Electric light and heat.

The electric spark was, of course, familiar to the early experimenters with electricity, but the electric light, as we know it, was first discovered by Sir Humphrey Davy, the Cornish philosopher, in the year 1811 or thereabout.  With the magic of his genius Davy transformed the spark into a brilliant glow by passing it between two points of carbon instead of metal.  If, as in figure 60, we twist the wires (+ and—­) which come from a voltaic battery, say of 20 cells, about two carbon pencils, and bring their tips together in order to start the current, then draw them a little apart, we shall produce an artificial or mimic star.  A sheet of dazzling light, which is called the electric arc, is seen to bridge the gap.  It is not a true flame, for there is little combustion, but rather a nebulous blaze of silvery lustre in a bluish veil of heated air.  The points of carbon are white-hot, and the positive is eaten away into a hollow or crater by the current, which violently tears its particles from their seat and whirls them into the fierce vortex of the arc.  The negative remains pointed, but it is also worn away about half as fast as the positive.  This wasting of the carbons tends to widen the arc too much and break the current, hence in arc lamps meant to yield the light for hours the sticks are made of a good length, and a self-acting mechanism feeds them forward to the arc as they are slowly consumed, thus maintaining the splendour of the illumination.

Many ingenious lamps have been devised by Serrin, Dubosq, Siemens, Brockie, and others, some regulating the arc by clockwork and electro-magnetism, or by thermal and other effects of the current.  They are chiefly used for lighting halls and railway stations, streets and open spaces, search-lights and lighthouses.  They are sometimes naked, but as a rule their brightness is tempered by globes of ground or opal glass.  In search-lights a parabolic mirror projects all the rays in any one direction, and in lighthouses the arc is placed in the focus of the condensing lenses, and the beam is visible for at least twenty or thirty miles on clear nights.  Very powerful arc lights, equivalent to hundreds of thousands of candles, can be seen for 100 or 150 miles.

Figure 61 illustrates the Pilsen lamp, in which the positive Carbon G runs on rollers rr through the hollow interior of two solenoids or coils of wire mm’ and carries at its middle a spindle-shaped piece of soft iron C. The current flows through the solenoid M on its way to the arc, but a branch or shunted portion of it flows through the solenoid M’, and as both of these solenoids act as electromagnets on the soft iron C, each tending to suck it into its interior, the iron rests between them when their powers are balanced.  When, however, the arc grows too wide, and the current therefore becomes too weak, the shunt solenoid M’ gains a purchase over the main solenoid M, and, pulling the iron core towards it, feeds the positive carbon to the arc.  In this way the balance of the solenoids is readjusted, the current regains its normal strength, the arc its proper width, and the light its brilliancy.