ALTERNATING CURRENTS. 285 



In this way we may measure discharges which would give 

 separately only very feeble swings. 



In order to demonstrate that the action of a current is pro- 

 portional to the quantity of electricity which passes, Pouillet* sent 

 through a galvanometer the succession of discharges obtained by 

 breaking periodically the circuit of a constant element. The break 

 consisted of a wooden wheel with a metal ring on its contour, which 

 was continuous on one side and cut away at equal intervals on 

 the other. Two elastic springs rest, one on the continuous and 

 the other on the grooved half. When this rests on a metal half 

 the current passes, and is broken when it is on the other. With 

 a short circuit and conductors without appreciable induction, the 

 deflection first increases with the velocity, then becomes constant 

 from a certain velocity, and proportional to the ratio between the 

 width of a tooth and the sum of a solid and of a hollow. In 

 Pouillet's experiments the currents could be broken 1200 times in 

 a second without the intensity undergoing any change; but this 

 law is no longer true if the effects of induction are not negligable, 

 and it is readily seen then that the intensity diminishes as the 

 velocity increases. 



Let I be the intensity of the permanent current, N the number 

 of breaks in a second, a the ratio of a solid to the sum of a solid 

 and of a hollow, the value of m determined by equation (54) gives 

 for the mean intensity 



i - e 



an expression which tends towards zero when N tends towards 

 infinity. 



The experiments of Bertinf and of Gazing on broken currents 

 are quite in agreement with this formula. It follows that the 

 quantity of electricity which corresponds to the extra current on 

 opening can really be neglected. 



900. ALTERNATING CURRENTS. When a circuit is traversed by 

 a series of transient currents or of discharges alternately in opposite 



* POUILLET. Comptes nndtts, Vol. iv., p. 787. 1837. 



t BERTIN. Ann. dc Chitn. et de Phys. [4], Vol. xvi., p. 25. 1869. 



CAZIX. Ann. de Ckim. et dc Phys. [4], Vol. XVII., p. 385. 1869. 



