Measurement of Small Capacities and Inductances. 509 



pointed out and guarded against by Prof. J. J. Thomson 

 (1883) and by Prof. J. J. Thomson and Mr. Searle (1890) in 

 making this class of measurement. 



Measurement of Inductance. — If two such commutators as 

 we have described were mounted on the same spindle, with 

 the eight radial teeth of each centre wheel (I in fig. 1) 

 insulated from each other, and a second central brush added 

 in the correct relative position to the first, we should obtain 

 an instrument similar in principle to the secohmmeter of 

 Professors Ayrton and Perry ; but we have thought it worth 

 while, in order to get a simpler apparatus, to sacrifice reversi- 

 bility and construct simply a double contact-breaker on the 

 same lines. By making the apparatus substantial and elimi- 

 nating all insulating material, except air, from the rotating 

 drums, and abolishing all flimsy spring-contacts, we have con- 

 structed an instrument which is much more satisfactory to work 

 with than the secohmmeter as made by the ordinary instru- 

 ment-maker (PL XII. fig. 5). This double contact-breaker con- 

 sists of a steel shaft which carries on it two circuit-interruptors 

 constructed in the following manner : — Each of these con- 

 sists of two wheels, like crown wheels, having four teeth, and 

 these two wheels are set on insulating bushes with the teeth 

 of one wheel interspaced between those of the other. The 

 shaft carries two such barrels (see fig. 5), and the developed 

 surface of these barrels is shown in fig. 6. The barrels are 

 formed of gun-metal, and against each barrel press two brass 

 gauze brushes carried on insulated brush-holders. One brush 

 bears on the continuous flange of one part of each barrel, and 

 the other brush alternately makes and breaks contact with it 

 by bearing on that part of the barrel occupied by the inter- 

 locked teeth. The two barrels can be set relatively to one 

 another in any position on the shaft. These two barrels serve 

 the purpose of making and breaking two separate electric 

 circuits in such fashion that at the moment when one circuit 

 is being broken, the other is complete, and at the moment 

 when the first circuit is being completed, the other is broken. 

 The arrangement is direct-driven through a flexible coupling 

 by a one-sixth H.P. direct-current shunt motor. 



In the measurement of inductance this double interrupter 

 is employed like the secohmmeter of Professors Ayrton and 

 Perry. The coil of which the inductance is to be determined 

 is balanced on a Wheatstone-bridge, and the two interrupters 

 of the rotating appliance are inserted respectively in the 

 battery and galvanometer circuits, so that if the apparatus 

 is set rotating after the steady balance of the non-inductive 

 resistance on the bridge has been obtained, it will eliminate 



Phil. Mag. 8. 6*. Vol. 5. No. 29. May 190.°). 2 M 



