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.
Ruhmkorff induction coils should always be fitted with a switch to open, close, or reverse the power circuit, a double throw, double pole, baby knife switch, mounted on a separate porcelain base, is very suitable. Such a switch is open when the handle is vertical, and it should always be left so when changing connections, fixing battery, etc. A large, well-finished coil will have the secondary wires brought in rubber tubes to binding posts mounted on hard rubber pillars, or to binding posts mounted considerably above the coil cover level. A very neat mode is shown in the frontispiece on the large 45-inch spark coil. Here the secondary wires go to hard rubber pillars, which also carry adjustable rod dischargers. These rods are movable towards or away from each other by means of the large hard rubber handle to which they are connected by a simple system of levers. In this coil the secondary is moulded on a flexible tube, which fits loosely over the primary tube in order to compensate for changes of temperature and consequent expansions and contractions. All well-designed coils should be so arranged that the primary coil and core can be readily removed from the secondary, or _vice versa_. It is sometimes desirable to use a different primary. This arrangement will greatly facilitate any necessary repairs. It must be always remembered that the working of a coil depends on the insulation between primary and secondary. _Spare no pains to have perfect insulation_; it is a hopeless task to reinsulate a broken-down secondary, although the sectional method of winding facilitates repairs. In large winding rooms it is customary to have a revolution counter connected to the spindle, so that the number of turns can be seen at all times. A bicycle cyclometer can be readily fitted up for this purpose, and will be found of considerable assistance where a number of sections are needed, each with a similar number of turns. In the commercial construction of telephone coils and magnet spools it is often the rule to specify only the number of turns of the requisite size wire, the ampere turns of the coils being thus regulated.