This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
This new instrument has excited considerable interest among telegraph and telephone men by its exceeding sensitiveness. It is so sensitive to the passage of an electric current that a battery formed with an ordinary pin for one electrode and a piece of zinc wire for the other, immersed in a single drop of water, will give sufficient current to operate the relay. In practice it has successfully worked as a telephonic call on the Eastern Railroad Company's line between Nancy and Paris, a distance of 212 miles, requiring but two cups of ordinary Leclanché battery.
The instrument consists of two permanent horseshoe magnets, fixed parallel with each other and an inch apart. A very thin spool or bobbin of insulated wire is suspended, like the pendulum of a clock, between these permanent magnets, in such a manner that the bobbin hangs just in front of the four poles. A counterpoise is fixed at the top of the pendulum bar, which permits the adjusting of the antagonistic forces represented by the action of the swinging bobbin, and two springs, which are insulated from the mass, and which form one electrode of the local or annunciator circuit, while the pendulum bar forms the other.
It will be easily understood that as the bobbin hangs freely in the center of a very strong magnetic field (formed by the four poles of the two permanent magnets), the slightest current sent through the bobbin will cause the bobbin to be attracted from one direction, while it will be repelled from the other, according to the polarity of the current transmitted.
As the relay has a very low resistance, it is evident that it will become an acceptable auxiliary in our central office, particularly when used as a "calling off" signal, as by its use the ground deviation, so objectionable and yet so universally used for "calling off" purposes, can be entirely avoided, and the relay left directly in the circuit, as is being done here in Paris. R. G. BROWN.
Paris, September 12, 1882.