Experiments with Alternating Currents, 251 



If there is a strong initial magnetization, we may assume 

 that the efficient pull on the wire will vary as the current 

 through the coils. 



Let F = efficient pull, 

 i = current, 

 then F oc i. 



Some rough calculations were made, and the conclusion 

 was arrived at that the self-induction of the electromagnets 

 was of the dimensions of 10 9 in C.Gr.S. units. 



The resistance of the primary circuit was about 3 x 10 9 

 C.G.S. units. 



Let T=time of vibration, 



n= frequency = ^, 



let i f = maximum current in the primary circuit, i. e. the 



current just before the break. 



Let it be assumed that the platinum point is in contact with 



T 

 the mercury for the time ~-. 



■pi 



Let ?o= ^, where E equals the E.M.F. and R the resist- 



ance of the primary circuit. 



The equation which represents the rise in the current after 

 contact is made is 



When t is small, 



t=i (l-e *>-) (1) 



*=*? CO 



Calculations showed that if T = 2, i f =i X '94667; T=^ 



1 1 5 > 



t/=*o x '2541 ; r l ,= gQ, i f =i x'03; T= ^, ^=2 X'003; 



T=jt^, i f =i x '0015 ; and that if w>50, formula (2) 



may, with very little error, be taken as correct. It can be 

 shown that the average current in the primary circuit varies 

 inversely as the frequency. 



The energy given to the vibrating wire in one complete 

 oscillation will now be considered. 



Since the current is stronger whilst the wire is rising than 

 when descending, the work done on the string by magnetic 

 forces when rising must be greater than the work done by the 

 string against magnetic forces when descending ; the spark will 

 also prolong the pull upwards. 



