474 Mr. W. E. Sumpner on the Variation 



wave, and the consequent current curve will, especially at 

 high speeds, differ in a very marked degree from the E.M.F. 

 curve of the machine. If the self-induction is a variable 

 quantity, the current curves obtained will be still more 

 rippled, and these ripples will be present at whatever speed 

 the dynamo is revolved. 



The well-known pulsations to which electric lights fed with 

 alternating currents are subject seem to suggest that the 

 currents obtained from the dynamos used are much less like 

 sine waves than is generally assumed to be the case. These 

 pulsations are so marked in the case of one important London 

 installation that any quickly moving object, such as a walking- 

 stick swayed rapidly to and fro presents, not a blurred image, 

 as it would do under a continuous light, but several distinctly 

 separate images, as if the light were at times very dim. The 

 pulsations are too slow to be due to the ordinary alternations 

 of the current, but might possibly be in time with the revo- 

 lutions of the dynamo. 



Condenser Discharges. 



19. It does not appear to have been noticed that self-induc- 

 tion, although always delaying the rise or fall of currents, may 

 sometimes hasten the discharge of a condenser. The current 

 which discharges a condenser has both to rise and fall. Self- 

 induction in the discharge-circuit delays the rise of current 

 less than the fall, because the potential-difference of the con- 

 denser is high when the current increases and low when it 

 diminishes ; the rate of change of current depends, not only 

 on the coefficient of self-induction, but also on the E.M.F. 

 tending to cause the change. The well-known oscillations 

 produced when the self-induction is large are simply due 

 to the fact that the current is kept flowing too long. The 

 general result is that the time of discharge is lessened if the 

 self-induction is not too great. 



If a condenser of capacity K, charged to a potential V , be 

 discharged by a wire having a resistance R and coefficient of 

 self-induction L, the potential of the condenser at any time t 

 is given by one of the equations 



v„ 



Ti-T 2 



Vn* 2L - 



;si n(ffl + 7) 

 sin 7 ' 



KR 2 



according respectively as L is less or greater than — j— . 



