Chapter VII: Front Matter (7)

Although countless forms of self-regulating device have been invented for arc lamps, nothing has survived the test of time so well as the typical mechanisms which work with carbon rods in one line, one or both rods being moved by a controlling apparatus as required. The early forms of semi-incandescent arc lamp, such as those of R. Werdermann and others, have dropped out of existence. These were not really true arc lamps, the light being produced by the incandescence of the extremity of a thin carbon rod pressed against a larger rod or block. The once famous Jablochkoff candle, invented in 1876, consisted of two carbon rods about 4 mm. in diameter, placed parallel to each other and separated by a partition of kaolin, steatite or other refractory non-conductor. Alternating currents were employed, and the candle was set in operation by a match or starter of high-resistance carbon paste which connected the tips of the rods. When this burned off, a true arc was formed between the parallel carbons, the separator volatilizing as the carbons burned away. Although much ingenuity was expended on this system of lighting between 1877 and 1881, it no longer exists. One cause of its disappearance was its relative inefficiency in light-giving power compared with other forms of carbon arc taking the same amount of power, and a second equally important reason was the waste in carbons. If the arc of the electric candle was accidentally blown out, no means of relighting existed; hence the great waste in half-burnt candles. H. Wilde, J. C. Jamin, J. Rapieff and others endeavoured to provide a remedy, but without success.

It is impossible to give here detailed descriptions of a fraction of
the arc-lamp mechanisms devised, and it must suffice to indicate the
broad distinctions between various types. (1) Arc lamps may be either
_continuous-current_ or _alternating-current_ lamps. For outdoor
public illumination the former are greatly preferable, as owing to the
form of the illuminating power-curve they send the light down on the
road surface, provided the upper carbon is the positive one. For
indoor, public room or factory lighting, _inverted arc_ lamps are
sometimes employed. In this case the positive carbon is the lower one,
and the lamp is carried in an inverted metallic reflector shield, so
that the light is chiefly thrown up on the ceiling, whence it is
diffused all round. The alternating-current arc is not only less
efficient in mean spherical candle-power per watt of electric power
absorbed, but its distribution of light is disadvantageous for street
purposes. Hence when arc lamps have to be worked off an
alternating-current circuit for public lighting it is now usual to
make use of a _rectifier_, which rectifies the alternating current
into an unidirectional though pulsating current. (2.) Arc lamps may be
also classified, as above described, into _open_ or _enclosed arcs_.
The enclosed arc can be made to burn for 200 hours with one pair of
carbons, whereas open-arc lamps are usually only able to work, 8, 16
or 32 hours without recarboning, even when fitted with double carbons.
(3) Arc lamps are further divided into _focussing_ and _non-focussing_
lamps. In the former the lower carbon is made to move up as the upper
carbon moves down, and the arc is therefore maintained at the same
level. This is advisable for arcs included in a globe, and absolutely
necessary in the case of lighthouse lamps and lamps for optical
purposes. (4) Another subdivision is into _hand-regulated_ and
_self-regulating_ lamps. In the hand-regulated arcs the carbons are
moved by a screw attachment as required, as in some forms of
search-light lamp and lamps for optical lanterns. The carbons in large
search-light lamps are usually placed horizontally. The
self-regulating lamps may be classified into groups depending upon the
nature of the regulating appliances. In some cases the regulation is
controlled only by a _series coil_, and in others only by a _shunt
coil_. Examples of the former are the original Gülcher and Brush
clutch lamp, and some modern enclosed arc lamps; and of the latter,
the Siemens "band" lamp, and the Jackson-Mensing lamp. In series coil
lamps the variation of the current in the coil throws into or out of
action the carbon-moving mechanism; in shunt coil lamps the variation
in voltage between the carbons is caused to effect the same changes.
Other types of lamp involve the use both of shunt and series coils
acting against each other. A further classification of the
self-regulating lamps may be found in the nature of the carbon-moving
mechanism. This may be some modification of the Brush ring clutch,
hence called _clutch_ lamps; or some variety of _brake wheel_, as
employed in Brockie and Crompton lamps; or else some form of _electric
motor_ is thrown into or out of action and effects the necessary
changes. In many cases the arc-lamp mechanism is provided with a
_dash-pot_, or contrivance in which a piston moving nearly air-tight
in a cylinder prevents sudden jerks in the motion of the mechanism,
and thus does away with the "hunting" or rapid up-and-down movements
to which some varieties of clutch mechanism are liable. One very
efficient form is illustrated in the Thomson lamp and Brush-Vienna
lamp. In this mechanism a shunt and series coil are placed side by
side, and have iron cores suspended to the ends of a rocking arm held
partly within them. Hence, according as the magnetic action of the
shunt or series coil prevails, the rocking arm is tilted backwards or
forwards. When the series coil is not in action the _motion_ is free,
and the upper carbon-holder slides down, or the lower one slides up,
and starts the arc. The series coil comes into action to withdraw the
carbons, and at the same time locks the mechanism. The shunt coil then
operates against the series coil, and between them the carbon is fed
forwards as required. The control to be obtained is such that the arc
shall never become so long as to flicker and become extinguished, when
the carbons would come together again with a rush, but the feed should
be smooth and steady, the position of the carbons responding quickly
to each change in the current.

The introduction of enclosed arc lamps was a great improvement, in
consequence of the economy effected in the consumption of carbon and
in the cost of labour for trimming. A well-known and widely used form
of enclosed arc lamp is the Jandus lamp, which in large current form
can be made to burn for two hundred hours without recarboning, and in
small or midget form to burn for forty hours, taking a current of two
amperes at 100 volts. Such lamps in many cases conveniently replace
large sizes of incandescent lamps, especially for shop lighting, as
they give a whiter light. Great improvements have also been made in
inclined carbon arc lamps. One reason for the relatively low
efficiency of the usual vertical rod arrangement is that the crater
can only radiate laterally, since owing to the position of the
negative carbon no crater light is thrown directly downwards. If,
however, the carbons are placed in a downwards slanting position at a
small angle like the letter V and the arc formed at the bottom tips,
then the crater can emit downwards all the light it produces. It is
found, however, that the arc is unsteady unless a suitable magnetic
field is employed to keep the arc in position at the carbon tips. This
method has been adopted in the Carbone arc, which, by the employment
of inclined carbons, and a suitable electromagnet to keep the true arc
steady at the ends of the carbons, has achieved considerable success.
One feature of the Carbone arc is the use of a relatively high voltage
between the carbons, their potential difference being as much as 85
volts.

Arrangement.

Arc lamps may be arranged either (i.) in series, (ii.) in parallel or (iii.) in series parallel. In the first case a number, say 20, may be traversed by the same current, in that case supplied at a pressure of 1000 volts. Each must have a magnetic cut-out, so that if the carbons stick together or remain apart the current to the other lamps is not interrupted, the function of such a cut-out being to close the main circuit immediately any one lamp ceases to pass current. Arc lamps worked in series are generally supplied with a current from a constant current dynamo, which maintains an invariable current of, say 10 amperes, independently of the number of lamps on the external circuit. If the lamps, however, are worked in series off a constant potential circuit, such as one supplying at the same time incandescent lamps, provision must be made by which a resistance coil can be substituted for any one lamp removed or short-circuited. When lamps are worked in parallel, each lamp is independent, but it is then necessary to add a resistance in series with the lamp. By special devices three lamps can be worked in series of 100 volt circuits. Alternating-current arc lamps can be worked off a high-tension circuit in parallel by providing each lamp with a small transformer. In some cases the alternating high-tension current is _rectified_ and supplied as a unidirectional current to lamps in series. If single alternating-current lamps have to be worked off a 100 volt alternating-circuit, each lamp must have in series with it a choking coil or economy coil, to reduce the circuit pressure to that required for one lamp. Alternating-current lamps take a larger _effective_ current, and work with a less effective or virtual carbon P.D., than continuous current arcs of the same wattage.

Cost.

The cost of working public arc lamps is made up of several items. There is first the cost of supplying the necessary electric energy, then the cost of carbons and the labour of recarboning, and, lastly, an item due to depreciation and repairs of the lamps. An ordinary type of open 10 ampere arc lamp, burning carbons 15 and 9 mm. in diameter for the positive and negative, and working every night of the year from dusk to dawn, uses about 600 ft. of carbons per annum. If the positive carbon is 18 mm. and the negative 12 mm., the consumption of each size of carbon is about 70 ft. per 1000 hours of burning. It may be roughly stated that at the present prices of plain open arc-lamp carbons the cost is about 15s. per 1000 hours of burning; hence if such a lamp is burnt every night from dusk to midnight the annual cost in that respect is about £1, 10s. The annual cost of labour per lamp for trimming is in Great Britain from £2 to £3; hence, approximately speaking, the cost per annum of maintenance of a public arc lamp burning every night from dusk to midnight is about £4 to £5, or perhaps £6, per annum, depreciation and repairs included. Since such a 10 ampere lamp uses half a Board of Trade unit of electric energy every hour, it will take 1000 Board of Trade units per annum, burning every night from dusk to midnight; and if this energy is supplied, say at 1½d. per unit, the annual cost of energy will be about £6, and the upkeep of the lamp, including carbons, labour for trimming and repairs, will be about £10 to £11 per annum. The cost for labour and carbons is considerably reduced by the employment of the enclosed arc lamp, but owing to the absorption of light produced by the inner enclosing globe, and the necessity for generally employing a second outer globe, there is a lower resultant candle-power per watt expended in the arc. Enclosed arc lamps are made to burn without attention for 200 hours, singly on 100 volt circuits, or two in series on 200 volt circuits, and in addition to the cost of carbons per hour being only about one-twentieth of that of the open arc, they have another advantage in the fact that there is a more uniform distribution of light on the road surface, because a greater proportion of light is thrown out horizontally.

It has been found by experience that the ordinary type of open arc lamp with vertical carbons included in an opalescent globe cannot compete in point of cost with modern improvements in gas lighting as a means of street illumination. The violet colour of the light and the sharp shadows, and particularly the non-illuminated area just beneath the lamp, are grave disadvantages. The high-pressure flame arc lamp with inclined chemically treated carbons has, however, put a different complexion on matters. Although the treated carbons cost more than the plain carbons, yet there is a great increase of emitted light, and a 9-ampere flame arc lamp supplied with electric energy at 1½d. per unit can be used for 1000 hours at an inclusive cost of about £s to £6, the mean emitted illumination being at the rate of 4 c.p. per watt absorbed. In the Carbone arc lamp, the carbons are worked at an angle of 15° or 20° to each other and the arc is formed at the lower ends. If the potential difference of the carbons is low, say only 50-60 volts, the crater forms between the tips of the carbons and is therefore more or less hidden. If, however, the voltage is increased to 90-100 then the true flame of the arc is longer and is curved, and the crater forms at the exteme tip of the carbons and throws all its light downwards. Hence results a far greater mean hemispherical candle power (M.H.S.C.P.), so that whereas a 10-ampere 60 volt open arc gives at most 1200 M.H.S.C.P., a Carbone 10-ampere 85 volt arc will give 2700 M.H.S.C.P. Better results still can be obtained with impregnated carbons. But the flame arcs with impregnated carbons cannot be enclosed, so the consumption of carbon is greater, and the carbons themselves are more costly, and leave a greater ash on burning; hence more trimming is required. They give a more pleasing effect for street lighting, and their golden yellow globe of light is more useful than an equally costly plain arc of the open type. This improvement in efficiency is, however, accompanied by some disadvantages. The flame arc is very sensitive to currents of air and therefore has to be shielded from draughts by putting it under an "economizer" or chamber of highly refractory material which surrounds the upper carbon, or both carbon tips, if the arc is formed with inclined carbons. (For additional information on flame arc lamps see a paper by L. B. Marks and H. E. Clifford, _Electrician_, 1906, 57, p. 975.)

2. _Incandescent Lamps._--Incandescent electric lighting, although not the first, is yet in one sense the most obvious method of utilizing electric energy for illumination. It was evolved from the early observed fact that a conductor is heated when traversed by an electric current, and that if it has a high resistance and a high melting-point it may be rendered incandescent, and therefore become a source of light. Naturally every inventor turned his attention to the employment of wires of refractory metals, such as platinum or alloys of platinum-iridium, &c., for the purpose of making an incandescent lamp. F. de Moleyns experimented in 1841, E. A. King and J. W. Starr in 1845, J. J. W. Watson in 1853, and W. E. Staite in 1848, but these inventors achieved no satisfactory result. Part of their want of success is attributable to the fact that the problem of the economical production of electric current by the dynamo machine had not then been solved. In 1878 T. A. Edison devised lamps in which a platinum wire was employed as the light-giving agent, carbon being made to adhere round it by pressure. Abandoning this, he next directed his attention to the construction of an "electric candle," consisting of a thin cylinder or rod formed of finely-divided metals, platinum, iridium, &c., mixed with refractory oxides, such as magnesia, or zirconia, lime, &c. This refractory body was placed in a closed vessel and heated by being traversed by an electric current. In a further improvement he proposed to use a block of refractory oxide, round which a bobbin of fine platinum or platinum-iridium wire was coiled. Every other inventor who worked at the problem of incandescent lighting seems to have followed nearly the same path of invention. Long before this date, however, the notion of employing carbon as a substance to be heated by the current had entered the minds of inventors; even in 1845 King had employed a small rod of plumbago as the substance to be heated. It was obvious, however, that carbon could only be so heated when in a space destitute of oxygen, and accordingly King placed his plumbago rod in a barometric vacuum. S. W. Konn in 1872, and S. A. Kosloff in 1875, followed in the same direction.

Carbon filament lamp.

No real success attended the efforts of inventors until it was finally recognized, as the outcome of the work by J. W. Swan, T. A. Edison, and, in a lesser degree, St. G. Lane Fox and W. E. Sawyer and A. Man, that the conditions of success were as follow: First, the substance to be heated must be carbon in the form of a thin wire rod or thread, technically termed a _filament_; second, this must be supported and enclosed in a vessel formed entirely of glass; third, the vessel must be exhausted as perfectly as possible; and fourth, the current must be conveyed into and out of the carbon filament by means of platinum wires hermetically sealed through the glass.

One great difficulty was the production of the carbon filament. King,
Sawyer, Man and others had attempted to cut out a suitably shaped
piece of carbon from a solid block; but Edison and Swan were the first
to show that the proper solution of the difficulty was to carbonize an
organic substance to which the necessary form had been previously
given. For this purpose cardboard, paper and ordinary thread were
originally employed, and even, according to Edison, a mixture of
lampblack and tar rolled out into a fine wire and bent into a spiral.
At one time Edison employed a filament of bamboo, carbonized after
being bent into a horse-shoe shape. Swan used a material formed by
treating ordinary crochet cotton-thread with dilute sulphuric acid,
the "parchmentized thread" thus produced being afterwards carbonized.
In the modern incandescent lamp the filament is generally constructed
by preparing first of all a form of soluble cellulose. Carefully
purified cotton-wool is dissolved in some solvent, such as a solution
of zinc chloride, and the viscous material so formed is forced by
hydraulic pressure through a die. The long thread thus obtained, when
hardened, is a semi-transparent substance resembling cat-gut, and when
carefully carbonized at a high temperature gives a very dense and
elastic form of carbon filament. It is cut into appropriate lengths,
which after being bent into horse-shoes, double-loops, or any other
shape desired, are tied or folded round carbon formers and immersed in
plumbago crucibles, packed in with finely divided plumbago. The
crucibles are then heated to a high temperature in an ordinary
combustion or electric furnace, whereby the organic matter is
destroyed, and a skeleton of carbon remains. The higher the
temperature at which this carbonization is conducted, the denser is
the resulting product. The filaments so prepared are sorted and
measured, and short leading-in wires of platinum are attached to their
ends by a carbon cement or by a carbon depositing process, carried out
by heating electrically the junction of the carbon and platinum under
the surface of a hydrocarbon liquid. They are then mounted in bulbs
of lead glass having the same coefficient of expansion as platinum,
through the walls of which, therefore, the platinum wires can be
hermetically sealed. The bulbs pass into the exhausting-room, where
they are exhausted by some form of mechanical or mercury pump. During
this process an electric current is sent through the filament to heat
it, in order to disengage the gases occluded in the carbon, and
exhaustion must be so perfect that no luminous glow appears within the
bulb when held in the hand and touched against one terminal of an
induction coil in operation.

In the course of manufacture a process is generally applied to the
carbon which is technically termed "treating." The carbon filament is
placed in a vessel surrounded by an atmosphere of hydrocarbon, such as
coal gas or vapour of benzol. If current is then passed through the
filament the hydrocarbon vapour is decomposed, and carbon is thrown
down upon the filament in the form of a lustrous and dense deposit
having an appearance like steel when seen under the microscope. This
deposited carbon is not only much more dense than ordinary carbonized
organic material, but it has a much lower specific electric
resistance. An untreated carbon filament is generally termed the
primary carbon, and a deposited carbon the secondary carbon. In the
process of treating, the greatest amount of deposit is at any places
of high resistance in the primary carbon, and hence it tends to cover
up or remedy the defects which may exist. The bright steely surface of
a well-treated filament is a worse radiator than the rougher black
surface of an untreated one; hence it does not require the expenditure
of so much electric power to bring it to the same temperature, and
probably on account of its greater density it ages much less rapidly.

Finally, the lamp is provided with a collar having two sole plates on
it, to which the terminal wires are attached, or else the terminal
wires are simply bent into two loops; in a third form, the Edison
screw terminal, it is provided with a central metal plate, to which
one end of the filament is connected, the other end being joined to a
screw collar. The collars and screws are formed of thin brass embedded
in plaster of Paris, or in some material like vitrite or black glass
(fig. 15). To put the lamp into connexion with the circuit supplying
the current, it has to be fitted into a socket or holder. Three of the
principal types of holder in use are the bottom contact (B.C.) or
Dornfeld socket, the Edison screw-collar socket and the Swan or loop
socket. In the socket of C. Dornfeld (fig. 16, a and a´) two spring
pistons, in contact with the two sides of the circuit, are fitted into
the bottom of a short metallic tube having bayonet joint slots cut in
the top. The brass collar on the lamp has two pins, by means of which
a bayonet connexion is made between it and the socket; and when this
is done, the spring pins are pressed against the sole plates on the
lamp. In the Edison socket (fig. 16, b) a short metal tube with an
insulating lining has on its interior a screw sleeve, which is in
connexion with one wire of the circuit; at the bottom of the tube, and
insulated from the screw sleeve, is a central metal button, which is
in connexion with the other side of the circuit. On screwing the lamp
into the socket, the screw collar of the lamp and the boss or plate at
the base of the lamp make contact with the corresponding parts of the
socket, and complete the connexion. In some cases a form of switch is
included in the socket, which is then termed the key-holder. For loop
lamps the socket consists of an insulated block, having on it two
little hooks, which engage with the eyes of the lamp. This insulating
block also carries some form of spiral spring or pair of spring loops,
by means of which the lamp is pressed away from the socket, and the
eyes kept tight by the hooks. This spring or Swan socket (fig. 16, c)
is found useful in places where the lamps are subject to vibration,
for in such cases the Edison screw collar cannot well be used, because
the vibration loosens the contact of the lamp in the socket. The
sockets may be fitted with appliances for holding ornamental shades or
conical reflectors.

The incandescent filament being a very brilliant line of light,
various devices are adopted for moderating its brilliancy and
distributing the light. A simple method is to sand-blast the exterior
of the bulb, whereby it acquires an appearance similar to that of
ground glass, or the bare lamp may be enclosed in a suitable glass
shade. Such shades, however, if made of opalescent or semi-opaque
glass, absorb 40 to 60% of the light; hence various forms of dioptric
shade have been invented, consisting of clear glass ruled with
prismatic grooves in such a manner as to diffuse the light without any
very great absorption. Invention has been fertile in devising etched,
coloured, opalescent, frosted and ornamental shades for decorative
purposes, and in constructing special forms for use in situations,
such as mines and factories for explosives, where the globe containing
the lamp must be air-tight. High candle-power lamps, 500, 1000 and
upwards, are made by placing in one large glass bulb a number of
carbon filaments arranged in parallel between two rings, which are
connected with the main leading-in wires. When incandescent lamps are
used for optical purposes it is necessary to compress the filament
into a small space, so as to bring it into the focus of a lens or
mirror. The filament is then coiled or crumpled up into a spiral or
zigzag form. Such lamps are called _focus lamps_.

Classification of lamps.

Incandescent lamps are technically divided into high and low voltage lamps, high and low efficiency lamps, standard and fancy lamps. The difference between high and low efficiency lamps is based upon the relation of the power absorbed by the lamp to the candle-power emitted. Every lamp when manufactured is marked with a certain figure, called the _marked volts_. This is understood to be the electromotive force in volts which must be applied to the lamp terminals to produce through the filament a current of such magnitude that the lamp will have a practically satisfactory life, and give in a horizontal direction a certain candle-power, which is also marked upon the glass. The numerical product of the current in amperes passing through the lamp, and the difference in potential of the terminals measured in volts, gives the total power taken up by the lamp in watts; and this number divided by the candle-power of the lamp (taking generally a horizontal direction) gives the _watts per candle-power_. This is an important figure, because it is determined by the temperature; it therefore determines the quality of the light emitted by the lamp, and also fixes the average duration of the filament when rendered incandescent by a current. Even in a good vacuum the filament is not permanent. Apart altogether from accidental defects, the carbon is slowly volatilized, and carbon molecules are also projected in straight lines from different portions of the filament. This process not only causes a change in the nature of the surface of the filament, but also a deposit of carbon on the interior of the bulb, whereby the glass is blackened and the candle-power of the lamp reduced. The volatilization increases very rapidly as the temperature rises. Hence at points of high resistance in the filament, more heat being generated, a higher temperature is attained, and the scattering of the carbon becomes very rapid; in such cases the filament is sooner or later cut through at the point of high resistance. In order that incandescent lighting may be practically possible, it is essential that the lamps shall have a certain _average life_, that is, duration; and this useful duration is fixed not merely by the possibility of passing a current through the lamp at all, but by the rate at which the candle-power diminishes. The decay of candle-power is called the _ageing_ of the lamp, and the useful life of the lamp may be said to be that period of its existence before it has deteriorated to a point when it gives only 75% of its original candle-power. It is found that in practice carbon filament lamps, as at present made, if worked at a higher efficiency than 2½ watts per candle-power, exhibit a rapid deterioration in candle-power and an abbreviated life. Hence lamp manufacturers classify lamps into various classes, marked for use say at 2½, 3, 3½ and 4 watts per candle. A 2½ watt per candle lamp would be called a _high-efficiency lamp_, and a 4 watt per candle lamp would be called a _low-efficiency_ lamp. In ordinary circumstances the low-efficiency lamp would probably have a longer life, but its light would be less suitable for many purposes of illumination in which colour discrimination is required.

The possibility of employing high-efficiency lamps depends greatly on the uniformity of the electric pressure of the supply. If the voltage is exceedingly uniform, then high-efficiency lamps can be satisfactorily employed; but they are not adapted for standing the variations in pressure which are liable to occur with public supply-stations, since, other things being equal, their filaments are less substantial. The classification into high and low voltage lamps is based upon the watts per candle-power corresponding to the marked volts. When incandescent lamps were first introduced, the ordinary working voltage was 50 or 100, but now a large number of public supply-stations furnish current to consumers at a pressure of 200 or 250 volts. This increase was necessitated by the enlarging area of supply in towns, and therefore the necessity for conveying through the same subterranean copper cables a large supply of electric energy without increasing the maximum current value and the size of the cables. This can only be done by employing a higher working electromotive force; hence arose a demand for incandescent lamps having marked volts of 200 and upwards, technically termed high-voltage lamps. The employment of higher pressures in public supply-stations has necessitated greater care in the selection of the lamp fittings, and in the manner of carrying out the wiring work. The advantages, however, of higher supply pressures, from the point of view of supply-stations, are undoubted. At the same time the consumer desired a lamp of a higher efficiency than the ordinary carbon filament lamp. The demand for this stimulated efforts to produce improved carbon lamps, and it was found that if the filament were exposed to a very high temperature, 3000° C. in an electric furnace, it became more refractory and was capable of burning in a lamp at an efficiency of 2½ watts per c.p. Inventors also turned their attention to substances other than carbon which can be rendered incandescent by the electric current.

Oxide filaments.

The luminous efficiency of any source of light, that is to say, the percentage of rays emitted which affect the eye as light compared with the total radiation, is dependent upon its temperature. In an ordinary oil lamp the luminous rays do not form much more than 3% of the total radiation. In the carbon-filament incandescent lamp, when worked at about 3 watts per candle, the luminous efficiency is about 5%; and in the arc lamp the radiation from the crater contains about 10 to 15% of eye-affecting radiation. The temperature of a carbon filament working at about 3 watts per candle is not far from the melting-point of platinum, that is to say, is nearly 1775° C. If it is worked at a higher efficiency, say 2.5 watts per candle-power, the temperature rises rapidly, and at the same time the volatilization and molecular scattering of the carbon is rapidly increased, so that the average duration of the lamp is very much shortened. An improvement, therefore, in the efficiency of the incandescent lamp can only be obtained by finding some substance which will endure heating to a higher temperature than the carbon filament. Inventors turned their attention many years ago, with this aim, to the refractory oxides and similar substances. Paul Jablochkoff in 1877 described and made a lamp consisting of a piece of kaolin, which was brought to a state of incandescence first by passing over it an electric spark, and afterwards maintained in a state of incandescence by a current of lower electromotive force. Lane Fox and Edison, in 1878, proposed to employ platinum wires covered with films of lime, magnesia, steatite, or with the rarer oxides, zirconia, thoria, &c.; and Lane Fox, in 1879, suggested as an incandescent substance a mixture of particles of carbon with the earthy oxides. These earthy oxides--magnesia, lime and the oxides of the rare earths, such as thoria, zirconia, erbia, yttria, &c.--possess the peculiarity that at ordinary temperatures they are practically non-conductors, but at very high temperatures their resistance at a certain point rapidly falls, and they become fairly good conductors. Hence if they can once be brought into a state of incandescence a current can pass through them and maintain them in that state. But at this temperature they give up oxygen to carbon; hence no mixtures of earthy oxides with carbon are permanent when heated, and failure has attended all attempts to use a carbon filament covered with such substances as thoria, zirconia or other of the rare oxides.

Nernst lamp.

H. W. Nernst in 1897, however, patented an incandescent lamp in which the incandescent body consists entirely of a slender rod or filament of magnesia. If such a rod is heated by the oxy-hydrogen blowpipe to a high temperature it becomes conductive, and can then be maintained in an intensely luminous condition by passing a current through it after the flame is withdrawn. Nernst found that by mixing together, in suitable proportions, oxides of the rare earths, he was able to prepare a material which can be formed into slender rods and threads, and which is rendered sufficiently conductive to pass a current with an electromotive force as low as 100 volts, merely by being heated for a few moments with a spirit lamp, or even by the radiation from a neighbouring platinum spiral brought to a state of incandescence.

The Nernst lamp, therefore (fig. 17), consists of a slender rod of the
mixed oxides attached to platinum wires by an oxide paste. Oxide
filaments of this description are not enclosed in an exhausted glass
vessel, and they can be brought, without risk of destruction, to a
temperature considerably higher than a carbon filament; hence the lamp
has a higher luminous efficiency. The material now used for the oxide
rod or "glower" of Nernst lamps is a mixture of zirconia and yttria,
made into a paste and squirted or pressed into slender rods. This
material is non-conductive when cold, but when slightly heated it
becomes conductive and then falls considerably in resistance. The
glower, which is straight in some types of the lamp but curved in
others, is generally about 3 or 4 cm. long and 1 or 2 mm. in diameter.
It is held in suitable terminals, and close to it, or round it, but
not touching it, is a loose coil of platinum wire, also covered with
oxide and called the "heater" (fig. 18). In series with it is a spiral
of iron wire, enclosed in a bulb full of hydrogen, which is called the
"ballast resistance." The socket also contains a switch controlled by
an electromagnet. When the current is first switched on it passes
through the heater coil which, becoming incandescent, by radiation
heats the glower until it becomes conductive. The glower then takes
current, becoming itself brilliantly incandescent, and the
electromagnet becoming energized switches the heater coil out of
circuit. The iron ballast wire increases in resistance with increase
of current, and so operates to keep the total current through the
glower constant in spite of small variations of circuit voltage. The
disadvantages of the lamp are (1) that it does not light immediately
after the current is switched on and is therefore not convenient for
domestic use; (2) that it cannot be made in small light units such as
5 c.p.; (3) that the socket and fixture are large and more complicated
than for the carbon filament lamp. But owing to the higher
temperature, the light is whiter than that of the carbon glow lamp,
and the efficiency or candle power per watt is greater. Since,
however, the lamp must be included in an opal globe, some considerable
part of this last advantage is lost. On the whole the lamp has found
its field of operation rather in external than in domestic lighting.

Metallic filament lamps.

Great efforts were made in the latter part of the 19th century and the first decade of the 20th to find a material for the filament of an incandescent lamp which could replace carbon and yet not require a preliminary heating like the oxide glowers. This resulted in the production of refractory metallic filament lamps made of osmium, tantalum, tungsten and other rare metals. Auer von Welsbach suggested the use of osmium. This metal cannot be drawn into wire on account of its brittleness, but it can be made into a filament by mixing the finely divided metal with an organic binding material which is carbonized in the usual way at a high temperature, the osmium particles then cohering. The difficulty has hitherto been to construct in this way metallic filament lamps of low candle power (16 c.p.) for 220 volt circuits, but this is being overcome. When used on modern supply circuits of 220 volts a number of lamps may be run in series, or a step-down transformer employed.

The next great improvement came when W. von Bolton produced the tantalum lamp in 1904. There are certain metals known to have a melting point about 2000° C. or upwards, and of these tantalum is one. It can be produced from the potassium tantalo-fluoride in a pulverulent form. By carefully melting it _in vacuo_ it can then be converted into the reguline form and drawn into wire. In this condition it has a density of 16.6 (water = 1), is harder than platinum and has greater tensile strength than steel, viz. 95 kilograms per sq. mm., the value for good steel being 70 to 80 kilograms per sq. mm. The electrical resistance at 15° C. is 0.146 ohms per metre with section of 1 sq. mm. after annealing at 1900° C. _in vacuo_ and therefore about 6 times that of mercury; the temperature coefficient is 0.3 per degree C. At the temperature assumed in an incandescent lamp when working at 1.5 watts per c.p. the resistance is 0.830 ohms per metre with a section of 1 sq. mm. The specific heat is 0.0365. Bolton invented methods of producing tantalum in the form of a long fine wire 0.05 mm. in diameter. To make a 25 c.p. lamp 650 mm., or about 2 ft., of this wire are wound backwards and forwards zigzag on metallic supports carried on a glass frame, which is sealed into an exhausted glass bulb. The tantalum lamp so made (fig. 19), working on a 110 volt circuit takes 0.36 amperes or 39 watts, and hence has an efficiency of about 1.6 watts per c.p. The useful life, that is the time in which it loses 20% of its initial candle power, is about 400-500 hours, but in general a life of 800-1000 hours can be obtained. The bulb blackens little in use, but the life is said to be shorter with alternating than with direct current. When worked on alternating current circuits the filament after a time breaks up into sections which become curiously sheared with respect to each other but still maintain electrical contact. The resistance of tantalum increases with the temperature; hence the temperature coefficient is positive, and sudden rises in working voltage do not cause such variations in candle-power as in the case of the carbon lamp.

Patents have also been taken out for lamps made with filaments of such infusible metals as tungsten and molybdenum, and Siemens and Halske, Sanders and others, have protected methods for employing zirconium and other rare metals. According to the patents of Sanders (German patents Nos. 133701, 137568, 137569) zirconium filaments are manufactured from the hydrogen or nitrogen compounds of the rare earths by the aid of some organic binding material. H. Kuzel of Vienna (British Patent No. 28154 of 1904) described methods of making metallic filaments from any metal. He employs the metals in a colloidal condition, either as hydrosol, organosol, gel, or colloidal suspension. The metals are thus obtained in a gelatinous form, and can be squirted into filaments which are dried and reduced to the metallic form by passing an electric current through them (_Electrician_, 57, 894). This process has a wide field of application, and enables the most refractory and infusible metals to be obtained in a metallic wire form. The zirconium and tungsten wire lamps are equal to or surpass the tantalum lamp in efficiency and are capable of giving light, with a useful commercial life, at an efficiency of about one watt per candle. Lamps called osram lamps, with filaments composed of an alloy of osmium and tungsten (wolfram), can be used with a life of 1000 hours when run at an efficiency of about 1.5 watts per candle.

Tungsten lamps are made by the processes of Just and Hanaman (German patent No. 154262 of 1903) and of Kuzel, and at a useful life of 1000 hours, with a falling off in light-giving power of only 10-15%, they have been found to work at an efficiency of one to 1.25 watts per c.p. Further collected information on modern metallic wire lamps and the patent literature thereof will be found in an article in the _Engineer_ for December 7, 1906.

Mention should also be made of the Helion filament glow lamp in which the glower is composed largely of silicon, a carbon filament being used as a base. This filament is said to have a number of interesting qualities and an efficiency of about 1 watt per candle (see the _Electrician_, 1907, 58, p. 567).

Mercury vapour lamps.

The mercury vapour lamps of P. Cooper-Hewitt, C. O. Bastian and others have a certain field of usefulness. If a glass tube, highly exhausted, contains mercury vapour and a mercury cathode and iron anode, a current can be passed through it under high electromotive force and will then be maintained when the voltage is reduced. The mercury vapour is rendered incandescent and glows with a brilliant greenish light which is highly actinic, but practically monochromatic, and is therefore not suitable for general illumination because it does not reveal objects in their daylight colours. It is, however, an exceedingly economical source of light. A 3-ampere Cooper-Hewitt mercury lamp has an efficiency of 0.15 to 0.33 watts per candle, or practically the same as an arc lamp, and will burn for several thousand hours. A similar lamp with mercury vapour included in a tube of _uviol_ glass specially transparent to ultra-violet light (prepared by Schott & Co. of Jena) seems likely to replace the Finsen arc lamp in the treatment of lupus. Many attempts have been made to render the mercury vapour lamp polychromatic by the use of amalgams of zinc, sodium and bismuth in place of pure mercury for the negative electrode.

Photometry of glow lamps.

An important matter in connexion with glow lamps is their photometry. The arrangement most suitable for the photometry and testing of incandescent lamps is a gallery or room large enough to be occupied by several workers, the walls being painted dead black. The photometer, preferably one of the Lummer-Brodhun form, is set up on a gallery or bench. On one side of it must be fixed a working standard, which as first suggested by Fleming is preferably a large bulb incandescent lamp with a specially "aged" filament. Its candle-power can be compared, at regular intervals and known voltages, with that of some accepted flame standard, such as the 10 candle pentane lamp of Vernon Harcourt. In a lamp factory or electrical laboratory it is convenient to have a number of such large bulb standard lamps. This working standard should be maintained at a fixed distance on one side of the photometer, such that when worked at a standard voltage it creates an illumination of one candle-foot on one side of the photometer disk. The incandescent lamp to be examined is then placed on the other side of the photometer disk on a travelling carriage, so that it can be moved to and fro. Arrangements must be made to measure the current and the voltage of this lamp under test, and this is most accurately accomplished by employing a potentiometer (q.v.). The holder which carries the lamp should allow the lamp to be held with its axis in any required position; in making normal measurements the lamp should have its axis vertical, the filament being so situated that none of the turns or loops overlies another as seen from the photometer disk. Observations can then be made of the candle-power corresponding to different currents and voltages.

The candle-power of the lamp varies with the other variables in
accordance with exponential laws of the following kind:--

If A is the current in amperes through the lamp, V the voltage or
terminal potential difference, W the power absorbed in watts, _c.p._
the maximum candle-power, and a, b, c, &c., constants, it has been
found that A and _c.p._ are connected by an exponential law such that

c.p. = aA^x

For carbon filament lamps x is a number lying between 5 and 6,
generally equal to 5.5 or 5.6. Also it has been found that c.p. = bW³
very nearly, and that

c.p. = cV^y nearly

where c is some other constant, and for carbon filaments y is a number
nearly equal to 6. It is obvious that if the candle-power of the lamp
varies very nearly as the 6th power of the current and of the voltage,
the candle-power must vary as the cube of the wattage.

Sir W. de W. Abney and E. R. Festing have also given a formula
connecting candle-power and watts equivalent to c.p. = (W - d)² where
d is a constant.

In the case of the tantalum lamp the exponent x has a value near to 6,
but the exponent y is a number near to 4, and the same for the osmium
filament. Hence for these metallic glowers a certain percentage
variation of voltage does not create so great a variation in
candle-power as in the case of the carbon lamp.

Curves delineating the relation of these variables for any
incandescent lamp are called its _characteristic-curves_. The life or
average duration is a function of W/c.p., or of the _watts per
candle-power_, and therefore of the voltage at which the lamp is
worked. It follows from the above relation that the watts per
candle-power vary inversely as the fourth power of the voltage.

From limited observations it seems that the average life of a
carbon-filament lamp varies as the fifth or sixth power of the watts
per candle-power. If V is the voltage at which the lamp is worked and
L is its average life, then L varies roughly as the twenty-fifth power
of the reciprocal of the voltage, or

L = aV^(-25).

A closer approximation to experience is given by the formula

V V²
log10L = 13.5 - -- - ------.
10 20,000

(See J. A. Fleming, "Characteristic Curves of Incandescent Lamps,"
_Phil. Mag._ May 1885).

Ageing of lamps.

All forms of incandescent or glow lamps are found to deteriorate in light-giving power with use. In the case of carbon filaments this is due to two causes. As already explained, carbon is scattered from the filament and deposited upon the glass, and changes also take place in the filament which cause it to become reduced in temperature, even when subjected to the same terminal voltage. In many lamps it is found that the first effect of running the lamp is slightly to increase its candle-power, even although the voltage be kept constant; this is the result of a small decrease in the resistance of the filament. The heating to which it is subjected slightly increases the density of the carbon at the outset; this has the effect of making the filament lower in resistance, and therefore it takes more current at a constant voltage. The greater part, however, of the subsequent decay in candle-power is due to the deposit of carbon upon the bulb, as shown by the fact that if the filament is taken out of the bulb and put into a new clean bulb the candle-power in the majority of cases returns to its original value. For every lamp there is a certain point in its career which may be called the "smashing-point," when the candle-power falls below a certain percentage of the original value, and when it is advantageous to replace it by a new one. Variations of pressure in the electric supply exercise a prejudicial effect upon the light-giving qualities of incandescent lamps. If glow lamps, nominally of 100 volts, are supplied from a public lighting-station, in the mains of which the pressure varies between 90 and 110 volts, their life will be greatly abbreviated, and they will become blackened much sooner than would be the case if the pressure were perfectly constant. Since the candle-power of the lamp varies very nearly as the fifth or sixth power of the voltage, it follows that a variation of 10% in the electromotive force creates a variation of nearly 50% in the candle-power. Thus a 16 candle-power glow lamp, marked for use at 100 volts, was found on test to give the following candle-powers at voltages varying between 90 and 105: At 105 volts it gave 22.8 c.p.; at 100 volts, 16.7 c.p.; at 95 volts, 12.2 c.p.; and at 90 volts, 8.7 c.p. Thus a variation of 25% in the candle-power was caused by a variation in voltage of only 5%. The same kind of variation in working voltage exercises also a marked effect upon the average duration of the lamp. The following figures show the results of some tests on typical 3.1 watt lamps run at voltages above the normal, taking the average life when worked at the marked volts (namely, 100) as 1000 hours:

At 101 volts the life was 818 hours.
" 102 " " 681 "
" 103 " " 662 "
" 104 " " 452 "
" 105 " " 374 "
" 106 " " 310 "

Voltage regulators.

Self-acting regulators have been devised by which the voltage at the points of consumption is kept constant, even although it varies at the point of generation. If, however, such a device is to be effective, it must operate very quickly, as even the momentary effect of increased pressure is felt by the lamp. It is only therefore where the working pressure can be kept exceedingly constant that high-efficiency lamps can be advantageously employed, otherwise the cost of lamp renewals more than counterbalances the economy in the cost of power. The slow changes that occur in the resistance of the filament make themselves evident by an increase in the watts per candle-power. The following table shows some typical figures indicating the results of ageing in a 16 candle-power carbon-filament glow lamp:--

+----------+-------------+-------------+
|Hours run.|Candle-Power.| Watts per |
| | |Candle-Power.|
+----------+-------------+-------------+
| 0 | 16.0 | 3.16 |
| 100 | 15.8 | 3.26 |
| 200 | 15.86 | 3.13 |
| 300 | 15.68 | 3.37 |
| 400 | 15.41 | 3.53 |
| 500 | 15.17 | 3.51 |
| 600 | 14.96 | 3.54 |
| 700 | 14.74 | 3.74 |
+----------+-------------+-------------+

The gradual increase in watts per candle-power shown by this table does not imply necessarily an increase in the total power taken by the lamp, but is the consequence of the decay in candle-power produced by the blackening of the lamp. Therefore, to estimate the value of an incandescent lamp the user must take into account not merely the price of the lamp and the initial watts per candle-power, but the rate of decay of the lamp.

Edison effect.

The scattering of carbon from the filament to the glass bulb produces interesting physical effects, which have been studied by T. A. Edison, W. H. Preece and J. A. Fleming. If into an ordinary carbon-filament glow lamp a platinum plate is sealed, not connected to the filament but attached to a third terminal, then it is found that when the lamp is worked with continuous current a galvanometer connected in between the middle plate and the positive terminal of the lamp indicates a current, but not when connected in between the negative terminal of the lamp and the middle plate. If the middle plate is placed between the legs of a horse-shoe-shaped filament, it becomes blackened most quickly on the side facing the negative leg. This effect, commonly called the _Edison effect_, is connected with an electric discharge and convection of carbon which takes place between the two extreme ends of the filament, and, as experiment seems to show, consists in the conveyance of an electric charge, either by carbon molecules or by bodies smaller than molecules. There is, however, an electric discharge between the ends of the filament, which rapidly increases with the temperature of the filament and the terminal voltage; hence one of the difficulties of manufacturing high-voltage glow lamps, that is to say, glow lamps for use on circuits having an electromotive force of 200 volts and upwards, is the discharge from one leg of the filament to the other.

Domestic use.