INVESTIGATION OF CURRENTS IN THE VARIABLE STATE. 2QQ 



the inducing current long enough to establish the permanent state, 

 and then breaks it ; after a variable interval of time the two wheels 

 B and C, by means of knife-edges with which they are fitted, connect 

 simultaneously, and for a time which does not exceed 0*000025 f a 

 second, a quadrant electrometer with the ends of the wire of the 

 induced coil. The electrometer acquires a permanent deflection 

 which measures the difference of potential of the two ends of the 

 wire at the moment of contact. The advantage of the electrometer 

 is that, whatever be its capacity, it quickly acquires its normal 

 charge although the contact is so short, and this charge does not 

 affect the distribution of potentials in the induced wire ; for when 

 the regime is once established, the electrometer is in a permanent 

 state, and the only effect of the successive contacts is to repair 

 the losses which may be rendered negligable. 



Observations agree in showing that the oscillations which follow 

 the first are isochronous. Their duration only depends on the 

 induced coil. That of the first oscillation, which is always longer, 

 depends further on the induced coil. It is greatly increased by 

 placing soft iron in the core of the secondary coil. 



913. The same method may be applied to the investigation 

 of alternating currents produced by induction machines. The make 

 and break is placed on the axis of the machine itself, and is ad- 

 justed so that the contact between the electrometer and any two 

 points of the induced wire, which is kept either open or closed, 

 takes place at a definite phase of the period. 



The phenomenon being made permanent, we might also use a 

 galvanometer. It is, however, to be feared that the shunt produced 

 by the galvanometer circuit, on making contact, would alter the 

 distribution of potentials on the induced wire. But this incon- 

 venience may be avoided by interposing in the circuit of the 

 galvanometer an electromotive force which can bring the needle 

 back to zero. It must be observed that the compensation is inde- 

 pendent of the manner in which the break acts, for it controls both 

 the current to be measured and that of the battery opposed to it. 



Fig. 176 shows the arrangement used. Let R be the resist- 

 ance between two points A and B of the principal circuit, Rj a 

 resistance shunting these two points, and AKGE a shunt of given 

 resistance, which, however, is very great, between the point A and 

 a variable point C of the resistance R, forming a resistance r be- 

 tween these two points. This shunt contains the galvanometer G, 



* JOUBERT. Ann. de V Ecok Normale [2], Vol. x., p. 145. 1882. 



