this coil is furnished with a little metal plate, and the other end of the wire rests loosely on this plate. If the keeper is placed over the poles, the flux emanating from them passes through the coil. If the keeper is taken away, the lines which previously passed through its interior vanish. By drawing the keeper away slowly the rate at which the lines vanish is slow, and consequently no very great e.m.f. will be induced in the coil, but if we accelerate this rate a fairly high e.m.f. may be induced. In order that it may be possible to tear off the keeper very quickly, it is provided with handles. By giving these handles a blow with both hands from below, the keeper comes off with a jerk, and the rate at which the flux diminishes is great, hence a large e.m.f. is induced. At the same time, as a consequence of the jerky motion, the point of the loose wire resting on the plate is caused to separate a little and thus an arc is produced as evidence that a current is circulating through the coil.
Some of the ignition apparatus used in motor cars and some mine exploders are constructed on the same principle. Faraday’s plate and loose wire are replaced by a properly constructed sparking. plug, but the spark is produced in the same way as in the original
