Part 14

It is a general rule that good conductors of electricity grow more resistant as the temperature rises while non-conductors resist less as the temperature rises. Hence the insulating material which is used to cover copper wires fails to protect if highly heated.

If a 110-volt lamp is put into a 220-volt circuit, one might expect that the lamp would burn out without doing further damage to the circuit, but this is not the case. As the filament approaches its melting point, 6000 degrees, it becomes so good a conductor that it carries current enough to melt a fifteen ampere fuse. It is, therefore, the fuse that protects the circuit and not the burning out of the lamp. The bulb containing the highly heated carbon vapour would conduct the current as an arc lamp does.

23. Arc Lamp.--We fastened two electric light carbons to the ends of copper wires connected for the 110-volt current. A rheostat, R (Fig. 99), in circuit, was set at 6.5 ohms. One lower carbon was fastened into a clamp, and the other was touched to it, and then drawn away about three-eighths of an inch. A very brilliant light was produced. Probably about 1800 candle-power. The ammeter A showed 10 amperes, and the volt meter V showed 45 volts. 45 volts × 10 amperes = 450 watts, 1800 candle-power, 25 watts per candle-power.

The arc light is the cheapest of all lights but is too dazzlingly bright for household purposes. It is used for outdoor lighting chiefly, and particularly for large search-lights. The temperature is over 6000 degrees, which boils the carbon and fills the gap between the two pencils with a stream of carbon vapour. This conducts the current like the filament in an incandescent lamp. The air gap between the carbon pencils would have a resistance of many thousand ohms if it were not for the presence of the carbon vapour. The hot carbon vapour reduces the resistance of this space to 4.5 ohms.

(45 volts)/(4.5 ohms) = 10 amperes.

(110 volts)/(6.5 + 4.5 ohms) = 10 amperes.

The carbon pencils account for part of this resistance--not more than a third of an ohm however.

It is evident that arc lamps in use must have an automatic mechanism which shall permit the carbons to touch whenever the current is not passing, but which shall draw them apart to the proper distance after the carbon vapour has been formed, or, as we say, after the arc has been established. This mechanism is nothing else than electro-magnets which are operated by the lighting circuit itself. It may require thoughtful examination to recognize these as electro-magnets, in every case, but that is what they are. Sometimes they are coils of wire, which do not have iron cores and armatures separate to be sure--but nevertheless they have both of these united in one movable rod, and they produce magnetic fields.

Suppose I pass an electric current around this coil A (Fig. 100). The region about the coil becomes a magnetic field with its north pole situated at a point in space, say N. The influence of this field causes the iron rod to become a magnet with its south pole uppermost, and if the current is strong enough, and the field which it produces is strong enough, it will lift the iron rod up into the coil. By varying the strength of the current you see I may make this rod dance up and down in space touching nothing--a veritable ghost dance.

It may be pettifogging to say that the upper portion of this iron rod is the core of the magnetic field, and its lower portion is the armature. Yet this is right, and pettifogging may be right when it is the only way to bring out the fact.

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.