Transactions of the Royal Society of Edinburgh . ntionthat when the current of three small Le dandie* cells, interrupted by a tuningfork, passes through the coils in opposite directions, nothing whatever can beheard, although one turn of the line wire outside the instrument restores thesound very distinctly, and the current through one coil produces a hum thatcan be heard a long way off. One of the simplest applications of the differential telephone is to thecomparison of coefficients of induction. In order to test this in practice, Ifitted up the following rough arrangement which I mean to re
Transactions of the Royal Society of Edinburgh . ntionthat when the current of three small Le dandie* cells, interrupted by a tuningfork, passes through the coils in opposite directions, nothing whatever can beheard, although one turn of the line wire outside the instrument restores thesound very distinctly, and the current through one coil produces a hum thatcan be heard a long way off. One of the simplest applications of the differential telephone is to thecomparison of coefficients of induction. In order to test this in practice, Ifitted up the following rough arrangement which I mean to replace by a better, after I have settled by experiencethe most convenient disposition ofthe parts, a/3, yS are two pairsof coils mounted on two stems, sothat they can be placed at anydistances apart up to 30 cm. or , CD are the terminals of thetwo coils of the differential tele-phone, EK is a multiple arc inone branch of which are /3a, andAB, in the other 8y and CD;the arrangement is such that the current passes in opposite directions round. PROFESSOR CHRYSTAL ON THE DIFFERENTIAL TELEPHONE. 611 the core in AB and CD, so that when the currents are equal at every instantthere is silence. In the single part of the line there is a battery G, and an interruptor F,which was sometimes a microphone attached to a clock, sometimes a tuningfork, but oftenest a piece of watch spring attached to the pendulum of a smallclock which grated over a milled head at the lowest part of each swing, andthus made a momentary contact. The last arrangement uses the least current,which is an advantage, but it also has the great virtue of being the most power-ful noise-producer that I have been able to find. The mathematical theory of the above arrangement, which is appended tothis paper, shows that there can be silence in the differential telephone when,and only when, the resistances of both branches of the multiple arc are equal,and also their coefficients of self-induction. It is of no consequence w
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