THE STRING GALVANOMETER 35 



corresponding contacts in the field magnet collar, 

 which are in turn connected with the sliding 

 contacts of the resistances already described. 



The weight of the " moving part " of the galva- 

 nometer is almost negligible, its moment of inertia 

 extremely small, and with the length of wires used 

 a period of swing as small as y^Jo^h of a second can 

 be obtained. The current flows in at C and 

 traverses the fine wires upwards from bottom to 

 top, leaving them at K. The field due to each is 

 thus similar, hence their displacement is lateral, and 

 is approximately equal to 



C (_ \ >< constant of galvanometer. 



\?i r 2 / 



We may regard the wires as elastic substances 

 stretched across the apparatus, where the dis- 

 placement D can be represented by f(e), or some 

 function of the elasticity ; the wires can be set in 

 vibration, and if one impulse be given by passing 

 a sudden short period current through the wires, 

 when displaced thereby through a distance D they 

 have sufficient potential energy to cause them to 

 swing back past the position of rest through a 

 distance slightly smaller than D in the same 

 manner as a pendulum. This is clearly shown by 

 the photographic records (p. no) obtained of the 

 movements of the shutter F recorded under various 

 circumstances on a moving photographic film. 

 This period can be made shorter by using shorter 



D 2 



