Fig. 30
The application of the electro-magnet to producing telegraphic signals will be understood by referring to [Fig. 30]. B is the generator of an electric current—sometimes a battery and sometimes a dynamo. One wire from this goes to the earth, E. The other wire goes through a key, which, like a push button or a switch, serves to open or close the circuit. This is normally closed when not in use. Through this the current passes around the electro-magnet S, which attracts the armature a, causing it to click against a metal stop, hence it is called the sounder. From this the current passes along the line wire to a distant station and there through the sounder and closed key to the earth. There is likely to be a generator at each station. The current must run continually through the system. If a battery is employed, the copper sulphate, or gravity cell, to be described later, is chosen, because it will endure continued usage better than any other.
The operator, in sending signals, opens the circuit, the magnets cease to hold down the armatures, and they are raised by springs and strike against metallic stops above. It is customary to say that the circuit is completed through the earth. This statement misleads some persons into imagining an electric current capable of corroding water pipes and decomposing chemical compounds, passing through the earth between stations.
Photograph by Helen W. Cooke
Testing the Telegraphy Outfit
Perhaps it will help to a better understanding of the truth if we think of a city pumping water out of the ocean, say to fight fire, and disposing of it again into the ocean. The ocean currents thus produced are not likely to be destructive. Indeed, just as we measure height from the ocean level as zero, so we measure electric pressures as from the zero level of the earth's electrical state.