Figure 62 is a diagrammatic representation of the Brush arc lamp.  X and Y are the line terminals connecting the lamp in circuit.  On the one hand, the current splits and passes around the hollow spools H H’, thence to the rod N through the carbon K, the arc, the carbon K’, and thence through the lamp frame to Y. On the other hand, it runs in a resistance fine-wire coil around the magnet T, thence to Y. The operation of the lamp is as follows:  K and K’ being in contact, a strong current starts through the lamp energising H and H’, which suck in their core pieces N and S, lifting C, and by it the “washer-clutch” W and the rod N and carbon K, establishing the arc.  K is lifted until the increasing resistance of the lengthening arc weakens the current in H H’ and a balance is established.  As the carbons burn away, C gradually lowers until a stop under W holds it horizontal and allows N to drop through W, and the lamp starts anew.  If for any reason the resistance of the lamp becomes too great, or the circuit is broken, the increased current through T draws up its armature, closing the contacts M, thus short-circuiting the lamp through a thick, heavy wire coil on T, which then keeps M closed, and prevents the dead lamp from interfering with the others on its line.  Numerous modifications of this lamp are in very general use.

Davy also found that a continuous wire or stick of carbon could be made white-hot by sending a sufficient current through it, and this fact is the basis of the incandescent lamp now so common in our homes.

Wires of platinum, iridium, and other inoxidisable metals raised to incandescence by the current are useful in firing mines, but they are not quite suitable for yielding a light, because at a very high temperature they begin to melt.  Every solid body becomes red-hot—­that is to say, emits rays of red light, at a temperature of about 1000 degrees Fahrenheit, yellow rays at 1300 degrees, blue rays at 1500 degrees, and white light at 2000 degrees.  It is found, however, that as the temperature of a wire is pushed beyond this figure the light emitted becomes far more brilliant than the increase of temperature would seem to warrant.  It therefore pays to elevate the temperature of the filament as high as possible.  Unfortunately the most refractory metals, such as platinum and alloys of platinum with iridium, fuse at a temperature of about 3450 degrees Fahrenheit.  Electricians have therefore forsaken metals, and fallen back on carbon for producing a light.  In 1845 Mr. Staite devised an incandescent lamp consisting of a fine rod or stick of carbon rendered white-hot by the current, and to preserve the carbon from burning in the atmosphere, he enclosed it in a glass bulb, from which the air was exhausted by an air pump.  Edison and Swan, in 1878, and subsequently, went a step further, and substituted a filament or fine thread of carbon for the rod.  The new lamp united the advantages of wire in point of form with those of carbon as a material.  The Edison filament was made by cutting thin slips of bamboo and charring them, the Swan by carbonising linen fibre with sulphuric acid.  It was subsequently found that a hard skin could be given to the filament by “flashing” it—­that is to say, heating it to incandescence by the current in an atmosphere of hydrocarbon gas.  The filament thus treated becomes dense and resilient.