Resistance Coils.
Although foreign to the title of this book, these coils will be mentioned, being often necessary as accessories to the operation of coils, wireless telegraphy, etc. These are coils of insulated German silver wire, wound to a specified resistance. The main feature about those designed for testing is that they are wound non-inductively—that is, the wire is wound double in such manner that the current flows both ways around the turns, and so neutralizes the inductive action. In cases where dynamo current is to be used, as in telegraphs operated from dynamo current, the coils are wound on tin tubes to make them fireproof and yet radiate the heat. As the resistance of German silver varies very largely, only approximate figures can be given. The table ([page 64]) has been made up from the best averages obtainable. The carrying capacity of resistance coils varies with their construction, the better they can radiate heat, the more current they can safely carry.