Chapter IV: Preface (4)

Our great study now is to produce light without heat, or at least to come as near to it as the firefly does. The firefly gives 98 per cent. light and two per cent. heat. The arc lamp gives 12 per cent. light and 88 per cent. heat. The carbon filament gives 4 per cent. light and 96 per cent. heat. When we have made considerable progress in that direction we shall take electric lamps out of the chapter on electric heating and form a new chapter on electric lighting.

One might expect that a rod made of carbon would quickly burn up, particularly when raised to the exceeding high temperature of the electric arc. While it is true that carbon in the form of charcoal burns so readily that it is used instead of kindlings for lighting a fire, carbon in the form of graphite in our so-called "lead" pencils and carbon as it is prepared for electric light pencils burns only very slowly even at exceedingly high temperatures. The carbon rods used in arc lamps endure a temperature of over 6000 degrees, without losing more than one inch an hour, and half of that is simply volatilized--not burned.

One of the most interesting improvements ever made in the arc light is that of enclosing the arc in an inner glass globe. This globe is closed airtight below with a small opening above. When the arc is formed the oxygen of the air in the inner globe is soon consumed and then combustion is no longer possible. We illustrated this by an experiment. An ordinary cork was chosen to fit the large end of an argand lamp chimney and through a hole in this was passed one of the carbon rods (Fig. 101). A metal clamp made connections between this carbon and the negative wire from the dynamo. The other carbon, attached by a clamp to the positive wire, was thrust down into the upper end of the chimney until it touched the negative carbon, and then drawn upward a short distance, drawing an arc, as we say. This soon makes an atmosphere within the chimney where combustion cannot go on for want of oxygen. The arc, however, continues to glow as in the open air, and the carbons may be drawn further apart than in the open air without breaking the arc, hence more of the external resistance may be cut out and a higher voltage put upon the lamp.

Carbons which burn out in a single night if used in open arc lamps last two weeks in enclosed arc lamps.

The lower carbon, when removed from the lamp chimney of the last experiment, served as a lead pencil to write on paper. The positive carbon would not make a mark on paper. In all arc lamps carbon is distilled from the positive pencil, condensing upon the negative pencil as graphite, which is the material used in making "lead" pencils. They are called "lead" pencils because they were originally made of lead, but now they are made of graphite which is mined from the earth.

As soon as the arc is broken it becomes evident that the positive carbon has been heated much the hotter of the two, a fact that could not be detected while it was lighted because of the dazzling brightness of the arc. The negative carbon turns black almost immediately, while the positive carbon remains at a bright red heat for some time.

This fact needs to be borne in mind when adjusting arc light carbons in search-lights, stereopticons, and all like apparatus in which the light must be placed at the focus of a lens. That is, it is necessary to know from what point the light really comes and it is necessary to have some adjusting device to keep this point continually at the focus of the lens.

24. _Search-Light._--(Fig. 102). This is simply an arc lamp with reflectors behind it and lenses in front of it. The whole apparatus is pivoted so as to be easily made to shine in any direction. The function of the lenses and the reflectors is to collect stray rays of light and send them all out in the same direction. This is shown in Fig. 103 where for simplicity the lens is represented as a single piece. _L_ represents a point of light which will naturally send its rays out in all directions as the radii of a sphere; _m_, _m_, _m_ represents a bright reflecting surface which is given that peculiar curve called a parabola. It has the unique faculty of reflecting in a parallel direction all the rays which may fall upon it from _L_, so long as _L_ is kept at that particular point called the focus, _a b_ is a lens of glass which has that peculiar curve that enables it to bend all rays which fall upon it from _L_, so that they may pass out parallel.

25. _Stereopticon._--This also has the necessary devices to gather the rays of the arc lamp and send them forth parallel, and in addition it has a series of lenses which produce upon a distant screen an enlarged picture of any transparent object held in these parallel rays.

26. _Burglar's Flash-Light._--There are many forms of this. The one we examined is represented in Fig. 104. We unscrewed a metal ring at the left-hand end and found, first a glass lens and behind that a miniature electric light, requiring three volts and half an ampere. We knew, therefore, that it must be supplied with two cells, since one cell may give not more than 1.5 volts. We also knew that it would only be used to _flash_ a light, since if dry cells are required to furnish half an ampere continuously they soon run down. Behind the lamp there was a bright metal reflector--the lens and reflector are fairly well represented in Fig. 103. The filament of the lamp is connected with two small battery cells in the handle. These may be removed and replaced by new ones by unscrewing a cap at the right-hand end. The circuit is closed by a metal spring on the side of the tube, which acts as a push button. It is situated where it may be conveniently pressed by the thumb. The small batteries necessarily have a short life and must be replaced quite frequently. Being a special thing they cost nearly twice what the regular dry cell does.

27. _Mercury Vapour Lamp._--This is an interesting variety of arc light in which the vapour of mercury takes the place of the vapour of carbon. _G_, in Fig. 105, represents a glass tube from which the air has been exhausted. The wires of the lighting circuit are fused into the ends of the tube. At one end, and in contact with one of these wires, is a small pool of mercury. By pulling the cord _c_ the tube is tilted on the pivot _p_, so that a stream of mercury flows along the whole length of the tube and closes the electric circuit. When the tube falls back into its normal position, as represented in the figure, the electric arc persists upon the mercury vapour. Incandescent mercury vapour gives light strong in green, blue, and violet, but deficient in red and yellow. It, therefore, gives nothing its natural appearance but casts a ghastly hue over everything.

This lamp was invented in 1902, by Peter Cooper-Hewitt, grandson of the founder of Cooper Union in New York City.

It gives a very suitable light for making photographic prints, and is much used for that. This lamp operates upon the 110-volt circuit. It is the longest step yet taken toward getting light without heat, but perhaps shows what we must expect when we reach that goal, namely, unsatisfactory colour values in the light. Probably such is the case with the firefly.

28. _The Moore Light._--In 1896 Prof. D. McFarland Moore brought out his vacuum tube light (Fig. 106). We visited an ordinary dry goods store which had been equipped with this. Glass tubing is put together very much as one would put up a stove pipe or a job of plumbing. The joints are fused and made air-tight by playing a flame upon them after the pipe is up in place. This pipe is led around into all nooks and corners where there would be dark places. The air is pumped out of this tube and a trifling amount of some vapour is introduced, the kind varying according to the tint of colour which is desired.

Metal terminals are fused into the ends of this tube. The tube we saw was seventy-five feet long. A 1000-volt alternating current is applied to the terminals and the vapour becomes incandescent, filling the whole tube full of light. The first thing that the boys remarked was that although the room was brilliantly lighted no object cast a shadow. It seemed as though light was everywhere and there was no chance to screen it off.

29. _The Nernst Lamp._--In 1897 the Nernst lamp appeared in Germany. It is a good illustration of an insulating substance becoming a conductor when heated to a high temperature. The "glower," as it is called, is composed of one or several short rods of clay-like material. This is first heated by sending the electric current through resistance wire placed directly underneath it and connected in shunt with it. When it gets hot, current begins to pass through it, and is automatically cut off from the resistance coil. The glower produces an intensely bright and white light although it does not itself exceed the temperature of 1742 degrees.

Electric installations are now so carefully constructed that fires from poor insulation are very rare. Less than one fire in three hundred appears to be traceable to that cause.

30. _Electric Welding._--Nothing is more common in electrical matters than heat produced by poor contacts. In this laboratory are two chandeliers, each controlled by a wall switch. After the current has been on the chandeliers for half an hour you will always find one of those wall switches warm, while the other is not perceptibly warmer than other objects in the room. The explanation is that there is poor contact in one of them. When two metal conductors touch one another at a mere point the electric current, in passing from one of these conductors to the other across such a narrow bridge, meets resistance and develops heat--sometimes heat enough to fuse the point, and either break the contact, or, what is more likely, start a minute arc at that point. In some cases this makes the apparatus dangerously hot, and in other cases it bridges the gap with a broader and better contact--a true electric weld. Electric welding is applied to everything, from chicken fence to railway rails. Enormously large currents are used for the purpose, in some cases as high as 50,000 amperes being employed. The rails of railroads are welded end to end by a current of several thousand amperes sent through the joint by perhaps two or three volts. The joint heats and fuses together merely because the poor contact offers resistance to this enormous current.

IX

LIGHTING A SUMMER CAMP BY ELECTRICITY

Summer had arrived. The Science Club had held its last meeting for the season. Harold had engaged three other boys to spend the summer at the farm. I had the roof of an old mill reshingled and gave it to them for a camp. They were to make it over inside. I sent the boys to the country as early as it was possible for them to get away. It would be six weeks later before I could follow them.

When I did arrive I found they had elaborate schemes indeed. The first floor of the mill had been partitioned off into rooms, as shown in diagram (Fig. 107), _a_, _b_, _c_ and _d_ being bedrooms; _e_ was a wash room, the like of which has never been seen before. It had not occurred to me that the mill pond _m_, which came to the very corner of the building, would furnish the boys a complete system of city water-works. At _g_, in the corner of this room, they had cut a hole in the floor and nailed slats across upon the under side of the timbers, making a depressed floor for a shower bath. This was directly over a stream of water which issued from the mill pond. Hanging from the ceiling over this spot was the nozzle of a garden hose. The other end of this hose ran into the mill pond. The nozzle was capable of delivering either a stream or a shower, according to which way it was twisted in its socket. It was also capable of shutting off entirely the flow of water. The boys asked me to hold my hand in the shower, and to my astonishment it was warm. "What, pray, is your heating system?" I inquired. They invited me to go and see. Moored outside in the mill pond at the corner of the building was our motor boat, which the boys were allowed to use freely and which they understood as well as any one.

They said that ordinarily they used for the shower the cool water of the lake, which they much preferred, and which ran of its own accord, the lake being a trifle higher than the nozzle of the shower, but knowing my antipathy for the cold bath they had slipped the end of the rubber hose over the outlet pipe of the pump which served to cool the gasolene engine in the boat. The engine uncoupled from the propeller was heating and pumping water for my shower bath, and I immediately accepted the invitation to enjoy it.

Certainly no bath was ever more delightful than that one, coming, as it did, at the close of a hot, dirty ride from the city.

I had hastened the bath, because it was already dusk and I had no candle at the mill, but suddenly the room lighted up as if by magic. I saw then what had before escaped my notice, a miniature electric lamp, six-volt, two-candle-power, tungsten, such as are used for tail lights on automobiles. Since tungsten requires about 1.25 watts per candle-power it was a 2.5-watts lamp, and since it was adapted to six volts it would take about four tenths of an ampere.

6 volts × .4 ampere = 2.4 watts. The little wire filament looked to be about 1.5 inches long. Its resistance must have been 15 ohms.

6 volts/15 ohms = .4 ampere.

A battery of five cells was used to furnish electric current for the lamp. Lamps were installed in the bedrooms also and were not intended to be used more than half an hour at a time. Dry battery cells are excellent for this purpose, and for so small a current the cheapest dry cells are as good as the more expensive ones. These cost fifteen cents a cell. They were connected by short pieces of bare copper wire; No. 18 "in series," as shown in Fig. 109. A wire ran from the central (carbon) binding post of one cell to the marginal (zinc) binding post of the next cell. This battery was placed on a shelf in a convenient place. A bare copper wire, No. 18, was attached to the carbon post at one end of the battery and another to the zinc post at the other end of the battery, and these two wires ran to all the rooms where lamps were placed. The wires were fastened up on the walls by staples, taking care that they should nowhere come in contact with each other and "short circuit" the battery. Whenever it was necessary for one wire to cross another, small pieces of pasteboard were tacked up to prevent their touching each other. The lamps _L_ (Fig. 109) were connected to these wires "in parallel." They cost forty cents apiece, and the miniature sockets, into which they were screwed, cost five cents each. One of these sockets was screwed to the side of the door casing in each bedroom. Wires were attached to the line wires, simply by twisting them together. One of these came down to one side of the socket and the other came to the other side of the socket through a switch, _s_, made of a strip of sheet zinc. The cost of the entire installation was as follows:

5 dry cells at 15c .75
5.2 cp., 6-volt tungsten lamps at 40c 2.00
5 miniature wall sockets at 5c .25
Wire, etc. .20
----
$3.20

Suppose each lamp is used thirty minutes a day for 100 days, making a total of fifty hours. There are five lamps, making a total of 250 lamp hours. Each lamp takes .4 of an ampere, making a total of 100 ampere hours. The lamps are operated at six volts, making a total of 600 watt hours.

100 days
.5 an hour each day
---
50 hours
5 lamps
---
250 lamp hours
.4 ampere for each lamp
---
100 ampere hours
6 volts
---
600 watt hours

This amount of electrical energy would cost six cents if generated by a dynamo. It is generally stated that electricity costs fifty times as much if generated by battery as by dynamo. In this case the battery actually did serve for the whole season of 100 days and was not exhausted at the end of the season.

Indeed, since that season, the boys have found that battery cells which had been too much exhausted for use on the engine served very well on the lamps. By use the cells lose, not much in voltage, but in the ability to furnish sufficient quantity in amperes to make the hot spark required for igniting the mixture of gasolene and air in an engine cylinder. When they have been discarded for use with the engine they may still furnish the small amount of current required for the lamps--provided not too many lamps are used at one time.

The dynamo current is always surprisingly cheap when compared with that produced by a battery, but, on the other hand, we are never as economical in the use of the dynamo current as we are with that of the battery.

If all five of the lamps in the above equipment were lighted at the same time and kept burning for half an hour, the battery would run down rather badly and would not fully recover. But if one only is used at a time and for not more than thirty minutes, or if more than one is used at a time and for a proportionately shorter period, the battery will receive no damage.

Dry battery cells may be purchased for either twenty-five cents or fifteen cents each. The chief difference is that the former are capable of giving larger current than the latter, when working against very small resistance. For example, the former may give twenty to twenty-five amperes on a short circuit, that is, connected directly with the ammeter without other resistance, while the latter may give not more than six to ten amperes under similar conditions. For most purposes, other than igniting gasolene engines, in which dry cells are used, an exceedingly small current is required. The electric bell, for example, may not require more than .2 of an ampere and that intermittently. Now it is found by experience that the dry cells which are only capable of furnishing on short circuit six to ten amperes will last quite as long in bell work as one which may give on short circuit twenty to twenty-five amperes. Hence it is good economy to buy them.

"What a fine sitting room you have here! (Fig. 107, _f._) When do you expect to fit it up?" said I. Instantly reminding myself, however, that boys do not want a sitting room, I inquired what they intended to use this fine, large room for. They told me that they had plans for making a machine shop out of that. The idea had been suggested by a counter shaft which still hung from the ceiling, and they had discovered that the old mill wheel would still roll over if the penstock were repaired. I replied that I would see what could be done about that sometime.

On the next day matters concerning the motor boat engaged our attention.

X

HOW ELECTRICITY FEELS

What is more fickle and yet more fascinating than a motor boat? On the morning after my arrival at Millville the boys wanted me to go out with them in the motor boat on the mill pond, as our beautiful little lake is called.

Each one took a hand at trying to start the boat, but although she had acted perfectly well the day before, on this morning no one could get a single explosion. The switch was closed. The gasolene was turned on. The carburetor valves were set at the mark. The spark coils responded with their familiar buzz. She had been primed and, when she had refused to respond to this treatment, the pet valves were opened and the wheel rolled over several times to sweep out the cylinders. But absolutely nothing moved her--neither coaxing nor gibes. Suddenly some one rolled the wheel over for the five-hundredth time and she started and behaved well all day.

All this would not have given us the slightest aggravation if we could only have found out what was the matter and what it was we finally did to correct it. But this we shall probably never know, and hence we are worshippers of the motor boat while we continue to distrust it and complain of it.

While the boat was running one of the boys noticed that a binding post at the end of one of the spark plugs seemed to be loose. He inadvertently put out his hand to tighten it and received a terrific shock. This raised the question among the boys, why one gets a shock from some of the binding posts in the electrical equipment but not from others. I suggested that we run in and call at the house to get my portable measuring instrument (Fig. 110) and a little lunch, and then go up to the upper end of the lake and take our time in examining the electrical equipment of the boat.

The engine had two cylinders. There were two batteries--one for each cylinder. Each battery consisted of five dry cells like the one represented in Fig. 111.

"Now, why don't I feel the electricity when I touch the binding posts of this dry cell?" inquired one of the boys as he handled one of the cells which we had taken out. "Well, I'll give you two reasons why do you not feel it," said I. "First, because you were touching only one binding post at a time. You must touch both of the binding posts of the battery cell at the same time, so that the electric current may pass from one post to the other through your body. Second, even when you do touch both binding posts at the same time you feel no current, simply because you offered probably about 100,000 ohms of resistance to the passage of the current and inasmuch as the one cell exerts only 1.5 volts of pressure, it could send only about .0000015 of an ampere through you. This you cannot feel.

(1.5 volts)/(100,000 ohms) = .0000015 amperes.

"I now connect my instrument as a volt meter between the binding posts of the cell and you see it indicates 1.5 volts, and when I connect it for an instant as an ammeter you see it indicates twenty amperes. That is twice as much as they use for executing criminals by electricity. So you see if you could reduce your resistance sufficiently this one battery cell might kill you. Some people have less resistance than others. The resistance of the body is chiefly in the outer skin. If one's hands are dry and his skin has been made tough and horny by hard work, he has many times the resistance of one whose hands are moist and whose skin is thin and tender.

"Suppose we select the tip of the tongue as the portion of the body which will offer the least resistance and will be most sensitive to slight electric currents. Let us then connect one dry cell with the ammeter and place the tip of the tongue between the bare ends of the wire at _T_ (Fig. 112).

"I have connected the ammeter so that it will indicate thousandths of an ampere, and you see that the needle moves only slightly. We cannot call it more than .001 ampere." Each boy in turn tried sending the current through his tongue and each tried to tell how it felt. One said it tingled, another said it felt warm, another said it tasted sour and the other said he did not feel or taste anything. "Well," I said, "whether you feel anything or not one-thousandth of an ampere is passing through your tongue and you are offering fifteen hundred ohms of resistance.

(1.5 volts)/(1500 ohms) = .001 ampere

"Your hand offers nearly seventy times as much resistance as your tongue. Suppose we try increasing the voltage, or pressure, of our electric current. We will connect in series the ten cells, making a battery which you see by the volt meter gives fifteen volts of pressure. We now find that having ten times the pressure it sends ten times as much current as formerly through the tongue."

(15 volts)/(1500 ohms) = .01 ampere

Each one now testified that the battery sent all the current he cared to take through his tongue. If they send one thousand times as much as that through a criminal no wonder it kills him. It produces a twitch when the contact is first made, afterward a decided sensation of warmth and acid taste.

If we should increase the voltage tenfold more, say the 110-volt dynamo current (direct current), and touch the bare conductors with our hands, the ammeter would indicate about .001 ampere. That is, although this current has about seventy times as much push, or voltage, as a dry cell, no more electricity passes through the fingers than did through the tongue in the preceding experiment with one cell. The fingers offer so much greater resistance.

By wetting the fingers and pressing them firmly upon the bare wires, we may make the ammeter read .01, that is, we may increase the current tenfold by reducing the resistance to one tenth. But there is nothing disagreeable about the feeling. If the same experiment is tried with the 110-volt alternating current, although the quantity of current which passes through the fingers is the same as before, the tingling is more perceptible than in the case of the direct current. If we join together seventy-five dry cells, giving a voltage of 112, and press the bare wires with our wet fingers, the ammeter will indicate .01, but there is no tingling sensation, merely a slight warmth. The battery current, being continuous, causes no twitching of the muscles while the contact is closed. The direct current dynamo furnishes a slightly pulsating current. Hence, one may tell by the feeling whether an electric current comes from a battery or a direct current dynamo. The alternating-current dynamo gives a surging of electricity back and forth in the wires, and this may be distinguished from the direct current by its feeling; when, however, the number of alternations per second is increased very greatly, one may receive through the body considerable quantities of electricity without feeling it. With a very high frequency current one may put himself in circuit and light a 16-candle-power lamp without any disagreeable sensation.

The outer skin is our chief insulation. If it is dry and well toughened by work it offers a resistance of over 100,000 ohms upon gentle contact. A wounded spot, or places like the tongue with moist, thin skin, may offer a resistance as low as 500 ohms. If one has a pin prick or a splinter in his hand which he cannot locate, he may hold one bare wire of a 110-volt alternating circuit in one hand and move the other bare wire about on the suspected region, and know when it reaches the spot by a tingling sensation.

One may touch lightly the 220-volt direct current and scarcely note any difference between this and the 110-volt direct current, because one is not very sensitive to the difference between .001 ampere and .002 ampere passing through his body.

(100 volts)/(100,000 ohms) = .001 ampere,
and
(200 volts)/(100,000 ohms) = .002 amperes

Physicians treat certain ailments by the use of the electric current. For this purpose they invariably use a pulsating or alternating current and reduce the resistance by using metal handles and wet sponges for contact with the skin, but even so a very small amount of current passes. The moderate twitching of the muscles seems to be the end sought.

Men who are supposed to be killed by electric shocks often die from other causes. A man perching upon an electric light pole, repairing wires, may come in contact with a wire charged, say, to 2000 volts. He may receive a shock which throws him in an unconscious condition across another live wire which burns its way into his flesh, or he may fall to the ground and be killed by the fall. A workman may hold a tool so as to short circuit a current through it, making it red hot in his hands. So many men who have been shocked into unconsciousness by high voltage currents have recovered consciousness later that we cannot say how much current is required to kill a man. For the execution of criminals 1800 to 2000 volts are used, and by special metal contacts ten to fourteen amperes are forced through the body.

The first execution of a criminal by electricity was performed in Sing Sing Prison, New York State, in 1890. There was at that time a hot controversy among experts over the question whether death, or merely unconsciousness, could be produced by electricity. To be on the safe side the legislature passed a law requiring that the electrocution of a criminal should be followed immediately by the dissection of his body. Only six states out of forty-nine have thus far adopted that method of capital punishment, five have abolished capital punishment, and thirty-eight still prefer hanging to electrocution. But it should be remembered that it is amperes, not volts, that kill. One often hears the meaningless expression, "he received 2000 volts into his body." The volts indicate the pressure, analogous to pounds per square inch of water pressure. Amperes of electricity are analogous to gallons of water. It is possible to have exceedingly high voltage of electricity without amperes enough to do damage. When one holds his finger near to a rapidly moving leather belt and a stream of sparks passes between the finger and the belt, the voltage may be 50,000 or even 100,000, but the quantity in amperes is too small to do any damage or even produce much sensation. A similar thing is true when one produces sparks by rubbing a cat's back, or lights the gas by a spark produced by rubbing the feet upon a carpet. Such sparks are miniature lightning discharges. The real lightning does damage because it furnishes quantity, measurable in amperes, as well as extremely high volts of pressure.

At this point I was reminded by the boy who had received a shock from the engine that morning that he had touched only one binding post. How then had he closed a circuit through his body, and how could he receive such a terrible shock when there were only a few battery cells to produce the electric current. I replied that he had the distinction of having encountered about a 5000-volt current. In the language of the newspapers he might say, _Took 5000 volts and still live._ We must next proceed to show how he really did close the circuit and how the spark coil enables a battery of a few dry cells to produce exceedingly high voltages.

XI

THE ELECTRICAL SPARKING EQUIPMENT FOR A GASOLENE ENGINE

Under the shade of a great sugar maple, with Millville Lake spread before us, we took apart and examined the entire equipment for producing the electric sparks to explode the mixture of gasolene and air in the cylinders of our motor boat. The engine has two cylinders. For each cylinder there is a separate battery and spark coil. Inasmuch as the electrical outfit is duplicated for each cylinder it will be necessary for us to consider the case of one cylinder only.

When this engine is running, 700 explosions per minute are produced in each cylinder. In one-twelfth of a second the following four events take place:

1. The cylinder is swept clear of the products of combustion formed by the last explosion.

2. Four drops of gasolene are vaporized and mixed with one quart of air and pushed into the cylinder by the pressure of the atmosphere.

3. This mixture is compressed by the piston in the cylinder to about one-fifth its original volume.

4. The mixture is heated to its kindling temperature, which is above 2000 degrees. It then burns with a sudden expansion, which drives the piston before it and pushes the crank which is concealed in the lower end of the cylinder half-way around. The crank is attached to the shaft, which carries the fly-wheel upon one end and the propeller wheel upon the other end. The momentum of the moving parts--chiefly that of the fly-wheel--suffices to accomplish the remaining half of the revolution.

That any machine could be devised which could repeat these four events 700 times a minute was unthinkable a few years ago.

The first men who thought that a gasolene engine could be a practical thing were considered visionaries, but now they are found to be more practicable than steam engines. They are so efficient that they compete with the steam engine upon its own ground, and, in addition, they have opened up regions of usefulness which the steam engine can never exploit. So far as we can see, they have a permanent monopoly of the navigation of the air.

It is with the fourth event mentioned above, viz., kindling the explosive mixture, that we are now concerned. The high temperature required for this is obtained by forcing an electrical current against resistance.

Five dry battery cells would very readily heat a short piece of fine wire to a sufficiently high temperature to explode the mixture, but it is impossible to alternately heat and cool a wire twelve times a second. It is too slow an operation. The only other method known at present is to imitate the lightning and force an electric current against the resistance of the air with sufficient power to produce the required heat. This, however, requires an extremely high voltage--at least 5000 volts, and our battery of five cells has not more than seven and a half volts of pressure. The interesting question then is, how does the spark coil enable us to raise the voltage from 7 to 5000.

To help toward an understanding of the matter I took seven small wire nails which I found in the boat--they were sixpenny finishing nails. I then took two or three yards of No. 24 insulated magnet wire, such as is used upon electric bells, etc. I use it more often than any other wire, and always have some about the boat. I fastened one end of this wire to one of the binding posts of a dry cell (Fig. 113), _a_, and attached branches _c_ and _d_ to it. The other end, _b_, was left free to act as a switch for closing the circuit by touching it to the remaining binding post.

One boy then touched the bare ends _c_ and _d_ to the tip of his tongue, while I touched repeatedly the binding post with _b_. There was, of course, no sensation. We now wound a portion of the wire upon the bundle of nails, laying on about fifty turns. (See Fig. 114.) The tongue was now placed at _T_ and _b_ was touched a few times to the free binding post. A very decided shock was felt, not while the end of the wire was resting upon _b_, but at the instant of touching and again at breaking the connection. The shock was noticeably stronger at the instant of breaking than of making the connection. There was also a spark formed when the connection was broken, which did not appear before the coil was made. We next wound on more of the wire--about fifty more turns (Fig. 115). When now connections were made and broken at _b_ the tongue at _T_ felt a much more decided shock, and a larger spark occurred at _b_ when the circuit was broken. Both the tongue and the spark indicate that the voltage is creeping up very rapidly in this series of experiments. We next connected two cells in series, then three, four, and finally five cells in place of the one. The spark grew larger and "fatter," as the boatmen say, with each addition of a cell. It was not pleasant to use the tongue in the experiment after the number of cells exceeded two. I removed the branch _d_ from the wire _b_ and connected it to the binding post, as shown in Fig. 116. I then removed the crystal from my watch and poured into it a little gasolene. I rubbed the ends of _b_ and _d_ together over this, and when they separated the spark which was produced would not light the gasolene. We had made a coil which produced a spark that looked like a miniature flame, but still was not hot enough to set fire to gasolene vapour. It simply needs more iron in the core and more turns of wire about it. Bringing the ends of the wires together and separating them is somewhat like drawing an arc with the arc light carbons. It requires a vastly higher voltage to make a spark jump across an air gap than it does to lead it across thus.

The kind of coil we have made (only larger) is very much used in houses as a gas-lighting coil (to be described later). It is very much used also for exploding gasolene engines. It generally passes under the name of the "make and break" coil. The revolving shaft of the engine is made to push together the ends of the wire and separate them at the right instant to make the spark for explosion. Of course this is done inside of the engine cylinder.

That type of coil does not offer resistance enough to protect the battery, and dry cells soon run down if used with it. The coils that we have in this boat are somewhat different from that, the details of which we cannot now entirely explain.

They offer enough resistance to cut the current required of the battery down to one third what the "make and break" coil would take and at the same time they raise the voltage so much higher that the spark will jump across an air gap without being led across as an arc. Hence they are called "jump spark" coils.

It will be remembered that when we were studying the dynamo we produced an electric current by moving a magnet. We may now add that an electric current may be produced by simply changing the strength of a magnetic field. The coil that we have just made creates a magnetic field in the region about itself whenever a current is passing through it. The tongue at _T_ (Fig. 117) detects an extra current while the magnetic field is being produced, or while it is dying away, or it will detect any slight variations in the strength of the current which produces the magnetic field. It is customary to distinguish between these two currents. The battery current which produced the magnetic field is called the primary current and the current which is detected by the tongue is called the secondary current. The primary current in our experiments had only a few volts of pressure, from one to seven. The secondary current had many volts, as indicated by the spark. If we rub the end of the wire _c_ across the binding post under _b_ (Fig. 117) no spark occurs. The current does not in this case go through the coil, and no secondary current is produced. Whenever we touch the wire _b_ to that post we have, in addition to the primary current which has not voltage enough to produce a spark, a secondary current flowing in the same wire at the same time and having voltage enough to produce a spark. The primary current is continuous while the contact is closed; the secondary current is momentary, as the tongue detects, and is produced only while changes are being made in the strength of the magnetic field. We will now take another piece of wire and wind upon the coil about two hundred more turns, leaving this outer coil wholly disconnected from the inner one, (Fig. 118). I connect _c_ and _d_, the terminals of what we may call the secondary coil, with my measuring instrument and I connect _a_, one of the terminals of the primary coil, with the battery. I then rub _b_, the other primary terminal across the free binding post of the battery. At the instant of closing the primary circuit--that is, of building up the magnetic field--a secondary current is induced in the secondary coil, which lasts for only an instant, too brief a time for the needle to measure it, although its motion indicates both the presence and the direction of the induced current. While the primary circuit remains closed--that is, while no change is occurring in the strength of the magnetic field--the needle returns to zero, indicating no secondary current. But when now the primary circuit is broken and the magnetic field loses its strength, the needle indicates a momentary current in the secondary coil and _in the opposite direction from what it had been at first_.

If, therefore, I rapidly make and break the current at _b_ I produce an alternating current in the secondary coil. I will connect _c_ and _d_ with a miniature lamp and, resting a coarse file upon the free binding post, I will rake the end of the wire _b_ up and down upon this file so that, as it dances along upon the file, it will rapidly make and break the primary circuit, and therefore rapidly change the strength of the magnetic field. You notice that the lamp lights up moderately well. It is being lighted by an alternating current. I move the wire a little more slowly and you see the flicker of the alternations. According to the label upon the lamp it requires ten volts, and our battery could not give that. We have therefore "stepped up" the voltage as we say and we have a veritable step-up transformer.

In this case the primary and secondary circuits are entirely separate. It is a familiar fact that different electric currents may pass through the same wire at the same time without apparent conflict. We send numerous telegraph despatches through the same wire at the same time. It is quite as easy for several pairs of persons to telephone over the same wire at the same time as it is for those same several pairs to carry on separate conversations in the same room at the same time, at, say, an "afternoon tea." We may use the same wire at the same time to carry direct and alternating currents. This fact was first discovered in 1902 by Bedell of Cornell University.

Primary and secondary currents do not require separate primary and secondary coils to convey them. They may or may not be connected into one continuous coil. It is quite immaterial whether they are connected or not so long as they are in the same magnetic field. Indeed, it seems that the field outside of the wire may be quite as important as the wire itself.

We have now 100 turns in the primary and 200 turns in the secondary coils. Let us connect _b_ with _c_ so as to make one continuous circuit of 300 turns. Let us then put a branch upon _b_ to connect with the battery, thus having 100 turns for the primary circuit, and put a branch upon _a_ to connect with the lamp, thus having 300 turns upon the lamp, (Fig. 119). When now we rub _b_ upon the file, as before, the lamp lights up more brightly than before, indicating that we have stepped up the voltage still higher. Varying the strength of the magnetic field induces a secondary current and the voltage of the induced current is determined, in part, by the number of turns in the secondary circuit. If what we have been saying is true we ought to be able to get these same results from an electric bell. To test this we connected wires with _a_ and _c_, (Fig. 120), and since I knew that the secondary current at _S_ would be too severe for the tongue we decided to feel it with the hands. For this purpose we want a larger surface than the wires themselves offer for contact with the hands, and so I twisted the bare end of each wire around an iron spike. The four boys then arranged themselves in line, joining hands, and the boy at each end of the line held a spike in his free hand. Thus we had put the enormous resistance of four human bodies joined in series in the secondary circuit. When now I connected two dry cells with _a_ and _b_ (_P_, Fig. 120) the hammer of the bell acted, like the file in the former case, as interrupter of the primary circuit. As it rapidly made and broke the primary circuit, it produced rapid changes in the strength of the magnetic field and thus induced a secondary current which the boys all felt. The fact that it forced its way through four bodies shows that its voltage was high. The high voltage was also indicated by the spark which always occurred in the bell. The primary circuit in this case has not more than three volts while the secondary has more than a hundred. We have it in our power to give the secondary current almost any voltage we choose, with this limitation _each increase in voltage necessitates a proportional sacrifice of quantity_. The watt power induced in the secondary circuit cannot exceed that contributed to the primary circuit--indeed cannot quite equal it since there is some loss in heat.

Suppose we operate a bell on a primary current having three volts and .25 ampere, that is, .75 watt. Suppose then the voltage of the secondary current is stepped up to fifty times three, or 150 volts. The quantity of secondary current will be found to be somewhat less than one fiftieth of .25 or .005 ampere. The 150-volt alternating current from the bell is more tolerable than that from a 150-volt dynamo, because the quantity is limited in the former case.

Our spark coil has a vibrator which acts precisely like the hammer of the bell to make and break the primary circuit and thus make rapid changes in the magnetic field produced by the primary coil. The primary coil of the spark coil is many times larger than the coil of the bell, that is, it contains many more turns of wire. It has much more iron in the core. We use upon it five cells instead of the two cells upon the bell. The result of all this is that we have a much more powerful magnetic field than that in the bell and many more watts of energy from which to induce a secondary current. Now the number of turns employed in the secondary circuit of our spark coil is very great, stepping its voltage up to thousands where the bell induced hundreds.

Suppose we now repeat our experiment in which we tried to light the gasolene in the watch crystal, using now the spark coil of the boat instead of our small "home-made" coil. In Fig. 121, B is the battery of five dry cells. _S_ is a switch. _V_ is the vibrator, which, like the hammer of an electric bell, makes and breaks the primary circuit. Of course the coil has a core of iron, although that is not here represented, and, of course, the coil has many hundred turns instead of the few here represented, and of course also it is built up of many layers instead of one as here represented. The secondary has very many more turns than the primary, but those in which the primary current passes are common to both circuits. There is also a condenser--not here represented, and not to be described in this book. The result of all this is that the secondary circuit has a voltage of between 5000 and 10,000, and a spark jumps across the gap at _c_ between one sixteenth and one eighth of an inch long. This spark is hot enough to light the gasolene which I have put in the watch crystal at _c_.

Let us return to the bell for a few minutes. I have here a miniature lamp which requires 10 volts and .1 ampere, that is, 1 watt, which I will connect at _S_ (Fig. 122). When now I close the primary circuit with two cells at _P_ you notice that the lamp lights up, but faintly. It is not receiving .1 ampere. Remember we have only .75 watt at our disposal and this lamp requires 1 watt. Hence it is getting only three quarters enough energy. We connect in a third cell and now it lights up to full brilliancy. The resistance of this lamp must be about 100 ohms.

(10 volts)/(100 ohms) = .1 ampere

The resistance of the four boys might have been 60,000 ohms, and the voltage of the secondary circuit might in that case have been, say, 150.

(150 volts)/(60,000 ohms) = .0052 ampere

How does it happen that the secondary current had a pressure of 150 volts on the boys but cannot supply even the 10 volts required by the lamp?

Perhaps we can be brought to appreciate the answer to that question best by asking ourselves some others quite like it.

Why did not the man who built our mill two generations ago locate it upon the small stream that flowed near his house? The small stream was more conveniently located for him and it has quite as much fall as he got at the foot of this lake. We sometimes express the fact by saying that the "head of water" or the water pressure was quite as much in one of these cases as the other.

One boy said that the stream sometimes gives out. Another one said that it never did have water enough to run that wheel. "Undoubtedly the trouble is with the quantity," said I, "but I want to show you that we cannot maintain the pressure unless there is sufficient quantity back of it."