54 Mr. J. J. Thomson on some Electromagnetic 



or 



F • 



— 8lD.pt 



.2 



LC P 



Now, if ™ be very much greater than p' 2 , this will be very 



much less than "~ -.- 8l f ^ , the expression for x when there is 



no condenser in the circuit, and will be of the opposite sign ; 

 so that the quantity of electricity travelling along the wire of 

 the coil at any time is very much less in this case than when 

 the circuit is "closed, and also the electricity moving along the 

 wire at any time is of opposite signs in the two cases. These 

 remarks will apply, I think, to the experiments last mentioned. 

 When we connect the breaking-points of the primary circuit 

 with the plates of a condenser, we produce an electrical oscil- 

 lation in the primary circuit whose period would be 2-zr\/L L C 1 

 if the secondary coil were not present, L L being the coefficient 

 of self-induction of the primary, and G 1 the capacity of the 

 condenser to the plates of which the breaking-points of the 

 primary are attached. These electrical oscillations in the pri- 

 mary will give rise to an electromotive force in the secondary 



which we may express by F sin — ; but the secondary 



circuit itself possesses both self-induction and electrostatic 

 capacity, and thus can be the seat of electrical oscillations 

 whose free period would be 27r\/L 2 C 2 , L 2 being the coeffi- 

 cient of self-induction of a current in the secondary coil, and 

 C 2 its electrostatic capacity. Now, since the currents in the 

 secondary are local, L 2 will not be the coefficient of self- 

 induction of the whole secondary circuit, but probably of only 

 a comparatively small part of it, so that L 2 will be much less 

 than Li; and as the condenser in the primary is of large 

 capacity and C 2 is only the electrostatic capacity of the coil 

 itself, C 2 will in all probability be much less than C lt Hence 

 27r\/L 2 C 2 will be very much less than 27p\/L 1 C 1 , or the 

 natural period of electrical vibrations in the secondary is very 

 much less than the period of the electromotive force acting 

 upon it. But the investigation we have just gone through 

 shows that in this case the amount of electricity passing along 

 the wire is very much less than it would be if the secondary 

 were closed, and that the electricity passing at any time is of 

 opposite signs in the two cases. This explains the great de- 

 crease in the magnetization of the needle, and the rnagnetiza- 



