Two men testing for bad joints in the railway. One man holds metal poles a and b three feet apart. Wires are connected to the poles that lead to the box the other man is holding. There is a bell inside of the box which will not ring when a current is running through it. When the phone makes a noise, there is a bad joint present.
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International Correspondence Schools A Textbook on Electric Lighting and Railways (Scranton, PA: International Textbook Company, 1901)