This lamp is essentially a series lamp; that is, any number of them can be put on one circuit wire, but a single lamp, used alone, burns equally well. It consists of a metal frame supporting at the bottom the holder for the globe and lower carbon, which is insulated from the frame.
The annexed figure of the plain lamp will convey an understanding of its general appearance. The upper carbon is fed downward by the mechanism contained in the box above, and is carried by a vertical round rod called the carbon holding rod.
THE THOMSON ARC LAMP.
In the regulating box of the lamp there exists a simple mechanism, the result of careful study and experiment to discover the best and simplest combination of appliances, which would obviate the necessity for the use of clockwork or dash-pots, from which fluids might be accidentally spilled, for obtaining a gradual feeding of the carbon as fast as it is consumed in producing the light, and at the same time to maintain the arc or space between the carbons in burning, of such extent as to give a steady, noiseless light, of greatest possible economy.
The lamp, once adjusted, does not require any readjustment, and, in fact, is built in such a manner as to avoid the presence of adjusting devices in it. The lamp also contains an automatic safety device for preserving the continuity of the circuit in case of accidental injury to the feeding mechanism or the carbons of the lamps. This is quite important when a considerable number of lights are operated upon one circuit wire, as a break in the circuit, due to a defective lamp, would result in the extinguishment of all the lights. With the safety device mentioned, such a break does not occur, but the flow of current is preserved through the faulty lamp.
By an exceedingly simple device upon the carbon holding rod, the lamps are extinguished when the carbons are burned out, and injury by burning the holders completely avoided.
The system is based upon the joint inventions of Elihu Thomson and Edwin J. Houston, for generators, regulators, and electric lamps, and also the patents of Elihu Thomson, in generators, regulators, and electric lamps; all of which are now operated and controlled by the Thomson-Houston Electric Co., 131 Devonshire Street, Boston, Mass.