is utilised in the well-known electric gas-lighters, where the whole of the mechanism diagrammatically represented in the sketch Fig. 4 is contained in the handle of the instrument. The rotation is produced by pressing a knob, and the cumulative action is vigorous enough to raise the potential of the two outer segments to sparking-point. The spark is produced at the end of a tube, which is held over the issuing gas-jet.
Let us then assume that there is a very feeble charge on A and B. If A is positive, a will receive a very small negative charge and b a very small positive charge. Let the rotation be clockwise. As the inner segments advance, the contact at the brushes is broken, and the negative charge of a is carried towards B. When the inner segment a has made a quarter turn, its contact piece touches the brush f, and thus the feeble charge is given up to B, making its charge just a little stronger than it was at starting. At the same time the feeble charge on b, which is positive, is given up to A, making also that charge a little stronger than it was. After half a turn from the start the segments a and b have changed places. They are again in contact by the brushes d e, and b acquires now a negative and a a positive
