Chapter VI: ARCHITECTURE, ART, ETC.--Artist's Homes. No. 11. "Weirleigh." (3)

[Here was shown a large electro-magnet and an induction-coil vacuum discharge spinning round and round when placed in its field.]

So much for what happens when electricity is made to travel along conductors, _i. e._, when it travels along like a stream of water in a pipe, or spins round and round like a whirlpool.

But there is another set of phenomena, usually regarded as distinct and of another order, but which are not so distinct as they appear, which manifest themselves when you join the pump to a piece of glass, or any non-conductor, and try to force the electricity through that. You succeed in driving some through, but the flow is no longer like that of water in an open pipe; it is as if the pipe were completely obstructed by a number of elastic partitions or diaphragms. The water can not move without straining and bending these diaphragms, and if you allow it, these strained partitions will recover themselves, and drive the water back again. [Here was explained the process of charging a Leyden jar.] The essential thing to remember is that we may have electrical energy in two forms, the static and the kinetic; and it is, therefore, also possible to have the rapid alternation from one of these forms to the other, called vibration.

Now we will pass to the second question: What do you mean by light? And the first and obvious answer is, Everybody knows. And everybody that is not blind does know to a certain extent. We have a special sense organ for appreciating light, whereas we have none for electricity. Nevertheless, we must admit that we really know very little about the intimate nature of light--very little more than about electricity. But we do know this, that light is a form of energy, and, moreover, that it is energy rapidly alternating between the static and the kinetic forms--that it is, in fact, a special kind of energy of vibration. We are absolutely certain that light is a periodic disturbance in some medium, periodic both in space and time; that is to say, the same appearances regularly recur at certain equal intervals of distance at the same time, and also present themselves at equal intervals of time at the same place; that in fact it belongs to the class of motions called by mathematicians undulatory or wave motions. The wave motion in this model (Powell's wave apparatus) results from the simple up and down motion popularly associated with the term wave. But when a mathematician calls a thing a wave he means that the disturbance is represented by a certain general type of formula, not that it is an up-and-down motion, or that it looks at all like those things on the top of the sea. The motion of the surface of the sea falls within that formula, and hence is a special variety of wave motion, and the term wave has acquired in popular use this signification and nothing else. So that when one speaks ordinarily of a wave or undulatory motion, one immediately thinks of something heaving up and down, or even perhaps of something breaking on the shore. But when we assert that the form of energy called light is undulatory, we by no means intend to assert that anything whatever is moving up and down, or that the motion, if we could see it, would be anything at all like what we are accustomed to in the ocean. The kind of motion is unknown; we are not even sure that there is anything like motion in the ordinary sense of the word at all.

Now, how much connection between electricity and light have we perceived in this glance into their natures? Not much, truly. It amounts to about this: That on the one hand electrical energy may exist in either of two forms--the static form, when insulators are electrically strained by having had electricity driven partially through them (as in the Leyden jar), which strain is a form of energy because of the tendency to discharge and do work; and the kinetic form, where electricity is moving bodily along through conductors or whirling round and round inside them, which motion of electricity is a form of energy, because the conductors and whirls can attract or repel each other and thereby do work.

And, on the other hand, that light is the rapid alternation of energy from one of these forms to the other--the static form where the medium is strained, to the kinetic form when it moves. It is just conceivable, then, that the static form of the energy of light is _electro_ static, that is, that the medium is _electrically_ strained, and that the kinetic form of the energy of light is _electro_-kinetic, that is, that the motion is not ordinary motion, but electrical motion--in fact, that light is an electrical vibration, not a material one.

On November 5, last year, there died at Cambridge a man in the full vigor of his faculties--such faculties as do not appear many times in a century--whose chief work has been the establishment of this very fact, the discovery of the link connecting light and electricity; and the proof--for I believe it amounts to a proof--that they are different manifestations of one and the same class of phenomena--that light is, in fact, an electro-magnetic disturbance. The premature death of James Clerk-Maxwell is a loss to science which appears at present utterly irreparable, for he was engaged in researches that no other man can hope as yet adequately to grasp and follow out; but fortunately it did not occur till he had published his book on "Electricity and Magnetism," one of those immortal productions which exalt one's idea of the mind of man, and which has been mentioned by competent critics in the same breath as the "Principia" itself.

But it is not perfect like the "Principia;" much of it is rough-hewn, and requires to be thoroughly worked out. It contains numerous misprints and errata, and part of the second volume is so difficult as to be almost unintelligible. Some, in fact, consists of notes written for private use and not intended for publication. It seems next to impossible now to mature a work silently for twenty or thirty years, as was done by Newton two and a half centuries ago. But a second edition was preparing, and much might have been improved in form if life had been spared to the illustrious author.

The main proof of the electro-magnetic theory of light is this: The rate at which light travels has been measured many times, and is pretty well known. The rate at which an electro-magnetic wave disturbance would travel if such could be generated (and Mr. Fitzgerald, of Dublin, thinks he has proved that it can not be generated directly by any known electrical means) can be also determined by calculation from electrical measurements. The two velocities agree exactly. This is the great physical constant known as the ratio V, which so many physicists have been measuring, and are likely to be measuring for some time to come.

Many and brilliant as were Maxwell's discoveries, not only in electricity, but also in the theory of the nature of gases, and in molecular science generally, I can not help thinking that if one of them is more striking and more full of future significance than the rest, it is the one I have just mentioned--the theory that light is an electrical phenomenon.

The first glimpse of this splendid generalization was caught in 1845, five and thirty years ago, by that prince of pure experimentalists, Michael Faraday. His reasons for suspecting some connection between electricity and light are not clear to us--in fact, they could not have been clear to him; but he seems to have felt a conviction that if he only tried long enough and sent all kinds of rays of light in all possible directions across electric and magnetic fields in all sorts of media, he must ultimately hit upon something. Well, this is very nearly what he did. With a sublime patience and perseverance which remind one of the way Kepler hunted down guess after guess in a different field of research, Faraday combined electricity, or magnetism, and light in all manner of ways, and at last he was rewarded with a result. And a most out-of-the-way result it seemed. First, you have to get a most powerful magnet and very strongly excite it; then you have to pierce its two poles with holes, in order that a beam of light may travel from one to the other along the lines of force; then, as ordinary light is no good, you must get a beam of plane polarized light, and send it between the poles. But still no result is obtained until, finally, you interpose a piece of a rare and out-of-the-way material, which Faraday had himself discovered and made--a kind of glass which contains borate of lead, and which is very heavy, or dense, and which must be perfectly annealed.

And now, when all these arrangements are completed, what is seen is simply this, that if an analyzer is arranged to stop the light and make the field quite dark before the magnet is excited, then directly the battery is connected and the magnet called into action, a faint and barely perceptible brightening of the field occurs, which will disappear if the analyzer be slightly rotated. [The experiment was then shown.] Now, no wonder that no one understood this result. Faraday himself did not understand it at all. He seems to have thought that the magnetic lines of force were rendered luminous, or that the light was magnetized; in fact, he was in a fog, and had no idea of its real significance. Nor had any one. Continental philosophers experienced some difficulty and several failures before they were able to repeat the experiment. It was, in fact, discovered too soon, and before the scientific world was ready to receive it, and it was reserved for Sir William Thomson briefly, but very clearly, to point out, and for Clerk-Maxwell more fully to develop, its most important consequences. [The principle of the experiment was then illustrated by the aid of a mechanical model.]

This is the fundamental experiment on which Clerk-Maxwell's theory of light is based; but of late years many fresh facts and relations between electricity and light have been discovered, and at the present time they are tumbling in in great numbers.

It was found by Faraday that many other transparent media besides heavy glass would show the phenomenon if placed between the poles, only in a less degree; and the very important observation that air itself exhibits the same phenomenon, though to an exceedingly small extent, has just been made by Kundt and Rontgen in Germany.

Dr. Kerr, of Glasgow, has extended the result to opaque bodies, and has shown that if light be passed through magnetized _iron_ its plane is rotated. The film of iron must be exceedingly thin, because of its opacity, and hence, though the intrinsic rotating power of iron is undoubtedly very great, the observed rotation is exceedingly small and difficult to observe; and it is only by a very remarkable patience and care and ingenuity that Dr. Kerr has obtained his result. Mr. Fitzgerald, of Dublin, has examined the question mathematically, and has shown that Maxwell's theory would have enabled Dr. Kerr's result to be predicted.

Another requirement of the theory is that bodies which are transparent to light must be insulators or non-conductors of electricity, and that conductors of electricity are necessarily opaque to light. Simple observation amply confirms this; metals are the best conductors, and are the most opaque bodies known. Insulators such as glass and crystals are transparent whenever they are sufficiently homogeneous, and the very remarkable researches of Prof. Graham Bell in the last few months have shown that even _ebonite_, one of the most opaque insulators to ordinary vision, is certainly transparent to some kinds of radiation, and transparent to no small degree.

[The reason why transparent bodies must insulate, and why conductors must be opaque, was here illustrated by mechanical models.]

A further consequence of the theory is that the velocity of light in a transparent medium will be affected by its electrical strain constant; in other words, that its refractive index will bear some close but not yet quite ascertained relation to its specific inductive capacity. Experiment has partially confirmed this, but the confirmation is as yet very incomplete. But there are a number of results not predicted by theory, and whose connection with the theory is not clearly made out. We have the fact that light falling on the platinum electrode of a voltameter generates a current, first observed, I think, by Sir W. R. Grove--at any rate, it is mentioned in his "Correlation of Forces"--extended by Becquerel and Robert Sabine to other substances, and now being extended to fluorescent and other bodies by Prof. Minchin. And finally--for I must be brief--we have the remarkable action of light on selenium. This fact was discovered accidentally by an assistant in the laboratory of Mr. Willoughby Smith, who noticed that a piece of selenium conducted electricity very much better when light was falling upon it than when it was in the dark. The light of a candle is sufficient, and instantaneously brings down the resistance to something like one-fifth of its original value.

I could show you these effects, but there is not much to see; it is an intensely interesting phenomenon, but its external manifestation is not striking--any more than Faraday's heavy glass experiment was.

This is the phenomenon which, as you know, has been utilized by Prof. Graham Bell in that most ingenious and striking invention, the photophone. By the kindness of Prof. Silvanus Thompson, I have a few slides to show the principle of the invention, and Mr. Shelford Bidwell has been kind enough to lend me his home-made photophone, which answers exceedingly well for short distances.

I have now trespassed long enough upon your patience, but I must just allude to what may very likely be the next striking popular discovery; and that is the transmission of light by electricity; I mean the transmission of such things as views and pictures by means of the electric wire. It has not yet been done, but it seems already theoretically possible, and it may very soon be practically accomplished.

* * * * *

INTERESTING ELECTRICAL RESEARCHES.

During the last six years Dr. Warren de la Rue has been investigating, in conjunction with Dr. Hugo Muller, the various and highly interesting phenomena which accompany the electric discharge. From time to time the results of their researches were communicated to the Royal Society, and appeared in its Proceedings. Early last year Dr. De la Rue being requested to bring the subject before the members of the Royal Institution, acceded to the pressing invitation of his colleagues and scientific friends. The discourse, which was necessarily long postponed on account of the preparations that had to be made, was finally given on Friday, the 21st of January, and was one of the most remarkable, from the elaborate nature of the experiments, ever delivered in the theater of that deservedly famous institution.

Owing to the great inconvenience of removing the battery from his laboratory, Dr. de la Rue, despite the great expenditure, directed Mr. S. Tisley to prepare, expressly for the lecture, a second series of 14,400 cells, and fit it up in the basement of the Royal Institution. The construction of this new battery occupied Mr. Tisley a whole year, while the charging of it extended over a fortnight.

The "de la Rue cell," if we may so call one of these elements, consists of a zinc rod, the lower portion of which is embedded in a solid electrolyte, viz., chloride of silver, with which are connected two flattened silver wires to serve as electrodes. When these are united and the silver chloride moistened, chemical action begins, and a weak but constant current is generated.

The electromotive force of such a cell is 1.03 volts, and a current equivalent to one volt passing through a resistance of one ohm was found to decompose 0.00146 grain of water in one second. The battery is divided into "cabinets," which hold from 1,200 to 2,160 small elements each. This facilitates removal, and also the detection of any fault that may occur.

It will be remembered that in 1808 Sir Humphry Davy constructed his battery of 2,000 cells, and thus succeeded in exalting the tiny spark obtained in closing the circuit into the luminous sheaf of the voltaic arc. He also observed that the spark passed even when the poles were separated by a distance varying from 1/40 to 1/30 of an inch. This appears to have been subsequently forgotten, as we find later physicists questioning the possibility of the spark leaping over any interpolar distance. Mr. J. P. Gassiot, of Clapham, demonstrated the inaccuracy of this opinion by constructing a battery of 3,000 Leclanché cells, which gave a spark of 0.025 inch; a similar number of "de la Rue" cells gives an 0.0564 inch spark. This considerable increase in potential is chiefly due to better insulation.

The great energy of this battery was illustrated by a variety of experiments. Thus, a large condenser, specially constructed by Messrs. Varley, and having a capacity equal to that of 6,485 large Leyden jars, was almost immediately charged by the current from 10,000 cells. Wires of various kinds, and from 9 inches to 29 inches in length, were instantly volatilized by the passage of the electricity thus stored up. The current induced in the secondary wire of a coil by the discharge of the condenser through the primary, was also sufficiently intense to deflagrate wires of considerable length and thickness.

It was with such power at his command that Dr. De la Rue proceeded to investigate several important electrical laws. He has found, for example, that the positive discharge is more intermittent than the negative, that the arc is always preceded by a streamer-like discharge, that its temperature is about 16,000 deg., and its length at the ordinary pressure of the atmosphere, when taken between two points, varies as the square of the number of cells. Thus, with a battery of 1,000 cells, the arc was 0.0051 inch, with 11,000 cells it increased to 0.62 inch. The same law was found to hold when the discharge took place between a point and a disk; it failed entirely, however, when the terminals were two disks.

It was also shown that the voltaic arc is not a phenomenon of conduction, but is essentially a disruptive discharge, the intervals between the passage of two successive static sparks being the time required for the battery to collect sufficient power to leap over the interposed resistance. This was further confirmed by the introduction of a condenser, when the intervals were perceptibly larger.

Faraday proved that the quantity of electricity necessary to produce a strong flash of lightning would result from the decomposition of a single grain of water, and Dr. de la Rue's experiments confirm this extraordinary statement. He has calculated that this quantity of electricity would be 5,000 times as great as the charge of his large condenser, and that a lightning flash a mile long would require the potential of 3,500,000 cells, that is to say, of 243 of his powerful batteries.

In experimenting with "vacuum" tubes, he has found that the discharge is also invariably disruptive. This is an important point, as many physicists speak and write of the phenomenon as one of conduction. Air, in every degree of tenuity, refuses to act as a conductor of electricity. These experiments show that the resistance of gaseous media diminishes with the pressure only up to a certain point, beyond which it rapidly increases. Thus, in the case of hydrogen, it diminishes up to 0.642 mm., 845 millionths; it then rises as the exhaustion proceeds, and at 0.00065 mm., 8.6 millionths, it requires as high a potential as at 21.7 mm., 28.553 millionths. At 0.00137 mm., 1.8 millionth, the current from 11,000 cells would not pass through a tube for which 430 cells sufficed at the pressure of minimum resistance. At a pressure of 0.0055 mm., 0.066 millionth, the highest exhaust obtained in any of the experiments, even a one-inch spark from an induction coil refused to pass. It was also ascertained that there is neither condensacian nor dilatation of the gas in contact with the terminals prior to the passage of the discharge.

These researches naturally led to some speculation about the conditions under which auroral phenomena may occur. Observers have variously stated the height at which the aurora borealis attains its greatest brilliancy as ranging between 124 and 281 miles. Dr. de la Rue's conclusions fix the upper limit at 124 miles, and that of maximum display at 37 miles, admitting also that the aurora may sometimes occur at an altitude of a few thousand feet.

The aurora was beautifully illustrated by a very large tube, in which the theoretical pressure was carefully maintained, the characteristic roseate tinge being readily produced and maintained.

In studying the stratifications observed in vacuum tubes, Dr. de la Rue finds that they originate at the positive pole, and that their steadiness may be regulated by the resistance in circuit, and that even when the least tremor cannot be detected by the eye, they are still produced by rapid pulsations which may be as frequent as ten millions per second.

Dr. de la Rue concluded his interesting discourse by exhibiting some of the finest tubes of his numerous and unsurpassed collection.--_Engineering_

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MEASURING ELECTROMOTIVE FORCE.

Coulomb's torsion balance has been adapted by M. Baille to the measurement of low electromotive forces in a very successful manner, and has been found preferable by him to the delicate electrometers of Sir W. Thomson. It is necessary to guard it from disturbances due to extraneous electric influences and the trembling of the ground. These can be eliminated completely by encircling the instrument in a metal case connected to earth, and mounting it on solid pillars in a still place. Heat also has a disturbing effect, and makes itself felt in the torsion of the fiber and the cage surrounding the lever. These effects are warded off by inclosing the instrument in a non-conducting jacket of wood shavings.

The apparatus of M. Baille consists of an annealed silver torsion wire of 2.70 meters long, and a lever 0.50 meter long, carrying at each extremity a ball of copper, gilded, and three centimeters in diameter. Similar balls are fixed at the corners of a square 20.5 meters in the side, and connected in diagonal pairs by fine wire. The lever placed at equal distances from the fixed balls communicates, by the medium of the torsion wire, with the positive pole of a battery, P, the other pole being to earth.

Owing to some unaccountable variations in the change of the lever or needle, M. Baille was obliged to measure the change at each observation. This was done by joining the + pole of the battery to the needle, and one pair of the fixed balls, and observing the deflection; then the deflection produced by the other balls was observed. This operation was repeated several times.

The battery, X, to be measured consisted of ten similar elements, and one pole of it was connected to the fixed balls, while the other pole was connected to the earth. The needle, of course, remained in contact with the + pole of the charging battery, P.

The deflections were read from a clear glass scale, placed at a distance of 3.30 meters from the needle, and the results worked out from Coulomb's static formula,

C a = (4 m m')/d², with

______________
/ sum((p/g) r²)
O = / -------------
\/ C

[TEX: O = \sqrt{\frac{\sum \frac{p}{g} r^2}{C}}]

In M. Baillie's experiments, O = 437³, and sum(pr²)= 32171.6 (centimeter grammes), the needle having been constructed of a geometrical form.

The following numbers represent the potential of an element of the battery--that is to say, the quantity of electricity that the pole of that battery spreads upon a sphere of one centimeter radius. They are expressed in units of electricity, the unit being the quantity of electricity which, acting upon a similar unit at a distance of one centimeter, produces a repulsion equal to one gramme:

Volta pile 0.03415 open circuit. Zinc, sulphate of copper, copper 0.02997 " Zinc, acidulated water, copper, sulphate of copper 0.03709 " Zinc, salt water, carbon peroxide of manganese 0.05282 " Zinc, salt water, platinum, chloride of platinum 0.05027 " Zinc, acidulated water, carbon nitric acid 0.06285 "

These results were obtained just upon charging the batteries, and are, therefore, slightly higher than the potentials given after the batteries became older. The sulphate of copper cells kept about their maximum value longest, but they showed variations of about 10 per cent.

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TELEPHONY BY THERMIC CURRENTS.

While in telephonic arrangements, based upon the principle of magnetic induction, a relatively considerable expenditure of force is required in order to set the tightly stretched membrane in vibration, in the so-called carbon telephones only a very feeble impulse is required to produce the differences in the current necessary for the transmission of sounds. In order to produce relatively strong currents, even in case of sound-action of a minimum strength, Franz Kröttlinger, of Vienna, has made an interesting experiment to use thermo electric currents for the transmission of sound to a distance. The apparatus which he has constructed is exceedingly simple. A current of hot air flowing from below upward is deflected more or less from its direction by the human voice. By its action an adjacent thermo-battery is excited, whose current passes through the spiral of an ordinary telephone, which serves as the receiving instrument. As a source of heat the inventor uses a common stearine candle, the flame of which is kept at one and the same level by means of a spring similar to those used in carriage lamps. On one side of the candle is a sheet metal voice funnel fixed upon a support, its mouth being covered with a movable sliding disk, fitted with a suitable number of small apertures. On the other side a similar support holds a funnel-shaped thermo-battery. The single bars of metal forming this battery are very thin, and of such a shape that they may cool as quickly as possible. Both the speaking-funnel and the battery can be made to approach, at will, to the stream of warm air rising up from the flame. The entire apparatus is inclosed in a tin case in such a manner that only the aperture of the voice-funnel and the polar clamps for securing the conducting wires appear on the outside. The inside of the case is suitably stayed to prevent vibration. On speaking into the mouth-piece of the funnel, the sound-waves occasion undulations in the column of hot air which are communicated to the thermo-battery, and in this manner corresponding differences are produced in the currents in the wires leading to the receiving instrument.--_Oesterreichische-Ungarische Post._

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THE TELECTROSCOPE.

By MONS. SENLECQ, of Ardres.

This apparatus, which is intended to transmit to a distance through a telegraphic wire pictures taken on the plate of a camera, was invented in the early part of 1877 by M. Senlecq, of Ardres. A description of the first specification submitted by M. Senlecq to M. du Moncel, member of the Paris Academy of Sciences, appeared in all the continental and American scientific journals. Since then the apparatus has everywhere occupied the attention of prominent electricians, who have striven to improve on it. Among these we may mention MM. Ayrton, Perry, Sawyer (of New York), Sargent (of Philadelphia), Brown (of London), Carey (of Boston), Tighe (of Pittsburg), and Graham Bell himself. Some experimenters have used many wires, bound together cable-wise, others one wire only. The result has been, on the one hand, confusion of conductors beyond a certain distance, with the absolute impossibility of obtaining perfect insulation; and, on the other hand, an utter want of synchronism. The unequal and slow sensitiveness of the selenium likewise obstructed the proper working of the apparatus. Now, without a relative simplicity in the arrangement of the conducting wires intended to convey to a distance the electric current with its variations of intensity, without a perfect and rapid synchronism acting concurrently with the luminous impressions, so as to insure the simultaneous action of transmitter and receiver, without, in fine, an increased sensitiveness in the selenium, the idea of the telectroscope could not be realized. M. Senlecq has fortunately surmounted most of these main obstacles, and we give to-day a description of the latest apparatus he has contrived.

TRANSMITTER.

A brass plate, A, whereon the rays of light impinge inside a camera, in their various forms and colors, from the external objects placed before the lens, the said plate being coated with selenium on the side intended to face the dark portion of the camera This brass plate has its entire surface perforated with small holes as near to one another as practicable. These holes are filled with selenium, heated, and then cooled very slowly, so as to obtain the maximum sensitiveness. A small brass wire passes through the selenium in each hole, without, however, touching the plate, on to the rectangular and vertical ebonite plate, B, Fig. 1, from under this plate at point, C. Thus, every wire passing through plate, A, has its point of contact above the plate, B, lengthwise. With this view the wires are clustered together when leaving the camera, and thence stretch to their corresponding points of contact on plate, B, along line, C C. The surface of brass, A, is in permanent contact with the positive pole of the battery (selenium). On each side of plate, B, are let in two brass rails, D and E, whereon the slide hereinafter described works.

Rail, E, communicates with the line wire intended to conduct the various light and shade vibrations. Rail, D, is connected with the battery wire. Along F are a number of points of contact corresponding with those along C C. These contacts help to work the apparatus, and to insure the perfect isochronism of the transmitter and receiver. These points of contact, though insulated one from the other on the surface of the plate, are all connected underneath with a wire coming from the positive pole of a special battery. This apparatus requires two batteries, as, in fact, do all autographic telegraphs--one for sending the current through the selenium, and one for working the receiver, etc. The different features of this important plate may, therefore, be summed up thus:

FIGURE 1.

D. Brass rail, grooved and connected with the line wire working the receiver.

F. Contacts connected underneath with a wire permanently connected with battery.

C. Contacts connected to insulated wires from selenium.

E. Brass rail, grooved, etc., like D.

RECEIVER.

A small slide, Fig. 2, having at one of its angles a very narrow piece of brass, separated in the middle by an insulating surface, used for setting the apparatus in rapid motion. This small slide has at the points, D D, a small groove fitting into the brass rails of plate, B, Fig. 1, whereby it can keep parallel on the two brass rails, D and E. Its insulator, B, Fig. 2, corresponds to the insulating interval between F and C, Fig. 1.

A, Fig. 3, circular disk, suspended vertically (made of ebonite or other insulating material). This disk is fixed. All round the inside of its circumference are contacts, connected underneath with the corresponding wires of the receiving apparatus. The wires coming from the seleniumized plate correspond symmetrically, one after the other, with the contacts of transmitter. They are connected in the like order with those of disk, A, and with those of receiver, so that the wire bearing the No. 5 from the selenium will correspond identically with like contact No. 5 of receiver.

D, Fig. 4, gutta percha or vulcanite insulating plate, through which pass numerous very fine platinum wires, each corresponding at its point of contact with those on the circular disk, A.

The receptive plate must be smaller than the plate whereon the light impinges. The design being thus reduced will be the more perfect from the dots formed by the passing currents being closer together.

B, zinc or iron or brass plate connected to earth. It comes in contact with chemically prepared paper, C, where the impression is to take place. It contributes to the impression by its contact with the chemically prepared paper.

In E, Fig. 3, at the center of the above described fixed plate is a metallic axis with small handle. On this axis revolves brass wheel, F, Fig. 5.

On handle, E, presses continuously the spring, H, Fig. 3, bringing the current coming from the selenium line. The cogged wheel in Fig. 5 has at a certain point of its circumference the sliding spring, O, Fig. 5, intended to slide as the wheel revolves over the different contacts of disk, A, Fig. 3.

This cogged wheel, Fig. 5, is turned, as in the dial telegraphs, by a rod working in and out under the successive movements of the electro-magnet, H, and of the counter spring. By means of this rod (which must be of a non-metallic material, so as not to divert the motive current), and of an elbow lever, this alternating movement is transmitted to a catch, G, which works up and down between the cogs, and answers the same purpose as the ordinary clock anchor.

This cogged wheel is worked by clockwork inclosed between two disks, and would rotate continuously were it not for the catch, G, working in and out of the cogs. Through this catch, G, the wheel is dependent on the movement of electro-magnet. This cogged wheel is a double one, consisting of two wheels coupled together, exactly similar one with the other, and so fixed that the cogs of the one correspond with the void between the cogs of the others. As the catch, G, moves down it frees a cog in first wheel, and both wheels begin to turn, but the second wheel is immediately checked by catch, G, and the movement ceases. A catch again works the two wheels, turn half a cog, and so on. Each wheel contains as many cogs as there are contacts on transmitter disk, consequently as many as on circular disk, A, Fig. 3, and on brass disk within camera.

Having now described the several parts of the apparatus, let us see how it works. All the contacts correspond one with the other, both on the side of selenium current and that of the motive current. Let us suppose that the slide of transmitter is on contact No. 10 for instance; the selenium current starting from No. 10 reaches contact 10 of rectangular transmitter, half the slide bearing on this point, as also on the parallel rail, communicates the current to said rail, thence to line, from the line to axis of cogged wheel, from axis to contact 10 of circular fixed disk, and thence to contact 10 of receiver. At each selenium contact of the rectangular disk there is a corresponding contact to the battery and electro-magnet. Now, on reaching contact 10 the intermission of the current has turned the wheel 10 cogs, and so brought the small contact, O, Fig. 5, on No. 10 of the fixed circular disk.

As may be seen, the synchronism of the apparatus could not be obtained in a more simple and complete mode--the rectangular transmitter being placed vertically, and the slide being of a certain weight to its fall from the first point of contact sufficient to carry it rapidly over the whole length of this transmitter.

The picture is, therefore, reproduced almost instantaneously; indeed, by using platinum wires on the receiver connected with the negative pole, by the incandescence of these wires according to the different degrees of electricity we can obtain a picture, of a fugitive kind, it is true, but yet so vivid that the impression on the retina does not fade during the relatively very brief space of time the slide occupies in traveling over all the contacts. A Ruhmkorff coil may also be employed for obtaining sparks in proportion to the current emitted. The apparatus is regulated in precisely the same way as dial telegraphs, starting always from first contact. The slide should, therefore, never be removed from the rectangular disk, whereon it is held by the grooves in the brass rails, into which it fits with but slight friction, without communicating any current to the line wires when not placed on points of contact.

* * * * *

[Continued from SUPPLEMENT No. 274, page 4368.]

THE VARIOUS MODES OF TRANSMITTING POWER TO A DISTANCE.

[Footnote: A paper lately read before the Institution of Mechanical Engineers.]

By ARTHUR ACHARD, of Geneva.

But allowing that the figure of 22 H. P., assumed for this power (the result in calculating the work with compressed air being 19 H. P.) may be somewhat incorrect, it is unlikely that this error can be so large that its correction could reduce the efficiency below 80 per cent. Messrs. Sautter and Lemonnier, who construct a number of compressors, on being consulted by the author, have written to say that they always confined themselves in estimating the power stored in the compressed air, and had never measured the gross power expended. Compressed air in passing along the pipe, assumed to be horizontal, which conveys it from the place of production to the place where it is to be used, experiences by friction a diminution of pressure, which represents a reduction in the mechanical power stored up, and consequently a loss of efficiency.

The loss of pressure in question can only be calculated conveniently on the hypothesis that it is very small, and the general formula,

p1 - p 4L
------- = ---- f(u),
[Delta] D

[TEX: \frac{p_1 - p}{\Delta} = \frac{4L}{D}f(u)]

is employed for the purpose, where D is the diameter of the pipe, assumed to be uniform, L the length of the pipe, p1 the pressure at the entrance, p the pressure at the farther end, u the velocity at which the compressed air travels, [Delta] its specific weight, and f(u) the friction per unit of length. In proportion as the air loses pressure its speed increases, while its specific weight diminishes; but the variations in pressure are assumed to be so small that u and [Delta] may be considered constant. As regards the quantity f(u), or the friction per unit of length, the natural law which regulates it is not known, audit can only be expressed by some empirical formula, which, while according sufficiently nearly with the facts, is suited for calculation. For this purpose the binomial formula, au + bu², or the simple formula, b1 u², is generally adopted; a b and b1 being coefficients deduced from experiment. The values, however, which are to be given to these coefficients are not constant, for they vary with the diameter of the pipe, and in particular, contrary to formerly received ideas, they vary according to its internal surface. The uncertainty in this respect is so great that it is not worth while, with a view to accuracy, to relinquish the great convenience which the simple formula, b1 u², offers. It would be better from this point of view to endeavor, as has been suggested, to render this formula more exact by the substitution of a fractional power in the place of the square, rather than to go through the long calculations necessitated by the use of the binomial au + bu². Accordingly, making use of the formula b1 u², the above equation becomes,

p1 - p 4L
------- = ---- b1 u²;
[Delta] D

[TEX: \frac{p_1 - p}{\Delta} = \frac{4L}{D} b_1 u^2]

or, introducing the discharge per second, Q, which is the usual figure supplied, and which is connected with the velocity by the relation, Q = ([pi] D² u)/4, we have

p1 - p 64 b1
------- = --------- L Q².
[Delta] [pi]² D^5

[TEX: \frac{p_1 - p}{\Delta} = \frac{64 b_1}{\pi^2 D^5} L Q^2]

Generally the pressure, p1, at the entrance is known, and the pressure, p, has to be found; it is then from p1 that the values of Q and [Delta] are calculated. In experiments where p1 and p are measured directly, in order to arrive at the value of the coefficient b1, Q and [Delta] would be calculated for the mean pressure ½(p1 + p). The values given to the coefficient b1 vary considerably, because, as stated above, it varies with the diameter, and also with the nature of the material of the pipe. It is generally admitted that it is independent of the pressure, and it is probable that within certain limits of pressure this hypothesis is in accordance with the truth.

D'Aubuisson gives for this case, in his _Traité d'Hydraulique_, a rather complicated formula, containing a constant deduced from experiment, whose value, according to a calculation made by the author, is approximately b1 = 0.0003. This constant was determined by taking the mean of experiments made with tin tubes of 0.0235 meter (15/16 in.), 0.05 meter (2 in.), and 0.10 meter (4 in.) diameter; and it was erroneously assumed that it was correct for all diameters and all substances.

M. Arson, engineer to the Paris Gas Company, published in 1867, in the _Mémoires de la Société des Ingénieurs Civils de France_, the results of some experiments on the loss of pressure in gas when passing through pipes. He employed cast-iron pipes of the ordinary type. He has represented the results of his experiments by the binomial formula, au + bu², and gives values for the coefficients a and b, which diminish with an increase in diameter, but would indicate greater losses of pressure than D'Aubuisson's formula. M. Deviller, in his _Rapport sur les travaux de percement du tunnel sous les Alpes_, states that the losses of pressure observed in the air pipe at the Mont Cenis Tunnel confirm the correctness of D'Aubuisson's formula; but his reasoning applies to too complicated a formula to be absolutely convincing.

Quite recently M. E. Stockalper, engineer-in-chief at the northern end of the St. Gothard Tunnel, has made some experiments on the air conduit of this tunnel, the results of which he has kindly furnished to the author. These lead to values for the coefficient b1 appreciably less than that which is contained implicitly in D'Aubuisson's formula. As he experimented on a rising pipe, it is necessary to introduce into the formula the difference of level, h, between the two ends; it then becomes

p1 - p 64 b1
------- = --------- L Q² + h.
[Delta] [pi]² D^5

[TEX: \frac{p_1 - p}{\Delta} = \frac{64 b_1}{\pi^2 D^5} L Q^2 + h]

The following are the details of the experiments: First series of experiments: Conduit consisting of cast or wrought iron pipes, joined by means of flanges, bolts, and gutta percha rings. D = 0.20 m. (8 in.); L = 4,600 m. (15,100 ft,); h= 26.77 m. (87 ft. 10 in.). 1st experiment: Q = 0.1860 cubic meter (6.57 cubic feet), at a pressure of ½(p1 + p), and a temperature of 22° Cent. (72° Fahr.); p1 = 5.60 atm., p =5.24 atm. Hence p1 - p = 0.36 atm.= 0.36 x 10,334 kilogrammes per square meter (2.116 lb. per square foot), whence we obtain b1=0.0001697. D'Aubuisson's formula would have given p1 - p = 0.626 atm.; and M. Arson's would have given p1 - p = 0.9316 atm. 2d experiment: Q = 0.1566 cubic meter (5.53 cubic feet), at a pressure of ½(p1 + p), and a temperature of 22° Cent. (72° Fahr.); p1 = 4.35 atm., p = 4.13 atm. Hence p1 - p = 0.22 atm. = 0.22 X 10,334 kilogrammes per square meter (2,116 lb. per square foot); whence we obtain b1 = 0.0001816. D'Aubuisson's formula would have given p1 - p = 0.347 atm; and M. Arson's would have given p1 - p = 0.5382 atm. 3d experiment: Q = 0.1495 cubic meter (5.28 cubic feet) at a pressure of ½(p1 + p) and a temperature 22° Cent. (72° Fahr.); p1 = 3.84 atm., p = 3.65 atm. Hence p1 - p = 0.19 atm. = 0.19 X 10,334 kilogrammes per square meter (2.116 lb. per square foot); whence we obtain B1 = 0.0001966. D'Aubuisson's formula would have given p1 - p = 0.284 atm., and M. Arson's would have given p1 - p = 0.4329 atm. Second series of experiments: Conduit composed of wrought-iron pipes, with joints as in the first experiments. D = 0.15 meter (6 in.), L - 0.522 meters (1,712 ft.), h = 3.04 meters (10 ft.) 1st experiments: Q = 0.2005 cubic meter (7.08 cubic feet), at a pressure of ½(p1 + p), and a temperature of 26.5° Cent. (80° Fahr.); p1 = 5.24 atm., p = 5.00 atm. Hence p1 - p = 0.24 atm. =0.24 x 10,334 kilogrammes per square meter (2,116 lb. per square foot); whence we obtain b1 = 0.3002275. 2nd experiment: Q = 0.1586 cubic meter (5.6 cubic feet), at a pressure of ½(p1 + p), and a temperature of 26.5° Cent. (80° Fahr.); p1 = 3.650 atm., p = 3.545 atm. Hence p1 - p = 0.105 atm. = 0.105 x 10,334 kilogrammes per square meter (2,116 lb. per square foot); whence we obtain b1 = 0.0002255. It is clear that these experiments give very small values for the coefficient. The divergence from the results which D'Aubuisson's formula would give is due to the fact that his formula was determined with very small pipes. It is probable that the coefficients corresponding to diameters of 0.15 meter (6 in.) and 0.20 meter (8 in.) for a substance as smooth as tin, would be still smaller respectively than the figures obtained above.

The divergence from the results obtained by M. Arson's formula does not arise from a difference in size, as this is taken into account. The author considers that it may be attributed to the fact that the pipes for the St. Gothard Tunnel were cast with much greater care than ordinary pipes, which rendered their surface smoother, and also to the fact that flanged joints produce much less irregularity in the internal surface than the ordinary spigot and faucet joints.

Lastly, the difference in the methods of observation and the errors which belong to them, must be taken into account. M. Stockalper, who experimented on great pressures, used metallic gauges, which are instruments on whose sensibility and correctness complete reliance cannot be placed; and moreover the standard manometer with which they were compared was one of the same kind. The author is not of opinion that the divergence is owing to the fact that M. Stockalper made his observations on an air conduit, where the pressure was much higher than in gas pipes. Indeed, it may be assumed that gases and liquids act in the same manner; and, as will be [1] explained later on, there is reason to believe that with the latter a rise of pressure increases the losses of pressure instead of diminishing them.

[Transcribers note 1: corrected from 'as will we explained']

All the pipes for supplying compressed air in tunnels and in headings of mines are left uncovered, and have flanged joints; which are advantages not merely as regards prevention of leakage, but also for facility of laying and of inspection. If a compressed air pipe had to be buried in the ground the flanged joint would lose a part of its advantages; but, nevertheless, the author considers that it would still be preferable to the ordinary joint.

It only remains to refer to the motors fed with the compressed air. This subject is still in its infancy from a practical point of view. In proportion as the air becomes hot by compression, so it cools by expansion, if the vessel containing it is impermeable to heat. Under these conditions it gives out in expanding a power appreciably less than if it retained its original temperature; besides which the fall of temperature may impede the working of the machine by freezing the vapor of water contained in the air.

If it is desired to utilize to the utmost the force stored up in the compressed air it is necessary to endeavor to supply heat to the air during expansion so as to keep its temperature constant. It would be possible to attain this object by the same means which prevent heating from compression, namely, by the circulation and injection of water. It would perhaps be necessary to employ a little larger quantity of water for injection, as the water, instead of acting by virtue both of its heat of vaporization and of its specific heat, can in this case act only by virtue of the latter. These methods might be employed without difficulty for air machines of some size. It would be more difficult to apply them to small household machines, in which simplicity is an essential element; and we must rest satisfied with imperfect methods, such as proximity to a stove, or the immersion of the cylinder in a tank of water. Consequently loss of power by cooling and by incomplete expansion cannot be avoided. The only way to diminish the relative amount of this loss is to employ compressed air at a pressure not exceeding three or four atmospheres.

The only real practical advance made in this matter is M. Mékarski's compressed air engine for tramways. In this engine the air is made to pass through a small boiler containing water at a temperature of about 120° Cent. (248° Fahr.), before entering the cylinder of the engine. It must be observed that in order to reduce the size of the reservoirs, which are carried on the locomotive, the air inside them must be very highly compressed; and that in going from the reservoir into the cylinder it passes through a reducing valve or expander, which keeps the pressure of admission at a definite figure, so that the locomotive can continue working so long as the supply of air contained in the reservoir has not come down to this limiting pressure. The air does not pass the expander until after it has gone through the boiler already mentioned. Therefore, if the temperature which it assumes in the boiler is 100° Cent. (212° Fahr.), and if the limiting pressure is 5 atm., the gas which enters the engine will be a mixture of air and water vapor at 100° Cent.; and of its total pressure the vapor of water will contribute I atm. and the air 4 atm. Thus this contrivance, by a small expenditure of fuel, enables the air to act expansively without injurious cooling, and even reduces the consumption of compressed air to an extent which compensates for part of the loss of power arising from the preliminary expansion which the air experiences before its admission into the engine. It is clear that this same contrivance, or what amounts to the same thing, a direct injection of steam, at a sufficient pressure, for the purpose of maintaining the expanding air at a constant temperature, might be tried in a fixed engine worked by compressed air with some chance of success.

Whatever method is adopted it would be advantageous that the losses of pressure in the pipes connecting the compressors with the motors should be reduced as much as possible, for in this case that loss would represent a loss of efficiency. If, on the other hand, owing to defective means of reheating, it is necessary to remain satisfied with a small amount of expansion, the loss of pressure in the pipe is unimportant, and has only the effect of transferring the limited expansion to a point a little lower on the scale of pressures. If W is the net disposable force on the shaft of the engine which works the compressor, v1 the volume of air at the compressor, p1. given by the compressor, and at the temperature of the surrounding air, and p0 the atmospheric pressure, the efficiency of the compressor, assuming the air to expand according to Boyle's law, is given by the well-known formula--

p1 v1 log (p1 / p0)
-------------------.
W

[TEX: \frac{p_1 v_1 \log \frac{p_1}{p_0}}{W}]

Let p2 be the value to which the pressure is reduced by the loss of pressure at the end of the conduit, and v2 the volume which the air occupies at this pressure and at the same temperature; the force stored up in the air at the end of its course through the conduit is p2 v2 log(p2/p0); consequently, the efficiency of the conduit is

p2 v2 log(p2/p0)
----------------
p1 v1 log(p1/p0)

[TEX: \frac{p_2 v_2 \log\frac{p_2}{p_0}}{p_2 v_2 \log\frac{p_2}{p_0}}]

a fraction that may be reduced to the simple form

log(p2/p0)
----------,
log(p1/p0)

[TEX: \frac{\log\frac{p_2}{p_0}}{\log\frac{p_2}{p_0}}]

if there is no leakage during the passage of the air, because in that cause p2 v2 = p1 v1. Lastly, if W1 is the net disposable force on the shaft of the compressed air motor, the efficiency of this engine will be,

W1
----------------
p2 v2 log(p2/p0)

[TEX: \frac{W_1}{p_2 v_2 \log \frac{p_2}{p_0}}]

and the product of these three partial efficiencies is equal to W1/W, the general efficiency of the transmission.