294 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



of the induction apparatus the primary circuit be first broken and then, 

 after the coil has had just one quarter enough time to reach its elonga- 

 tion, closed in reverse direction, the angular deviation of the coil will 

 be given as a function of pt by the curve OBMVJC, Figure E. The 

 ordinates of this curve are the sums of the corresponding ordinates of 

 OBDL and ADK. If the current in the primary circuit of the induction 

 apparatus were suddenly reversed while the galvanometer coil was at 

 rest in its position of equilibrium, the deviation would be given by the 



Figure E. 



curve OWFPH, the ordinates of which are double those of the curve 

 OBDL. The throw with the lag OA is nearly 99 per cent of that when 

 the current is suddenly reversed. 



This graphical process is especially convenient when the allowable 

 decrease of throw is given and one wishes to find the maximum lag 

 which will not make the throw difference too great. If the lag is given 

 and the throw difference is wanted, this may be found by computation, 

 though the graphical treatment has solid advantages. It is evident 

 that the curve y = e"^ • sin x serves for a given galvanometer with a 

 given coil circuit for throws of all magnitudes. 



It often happens that one has to work with a galvanometer the 

 period of which is rather too short for the purpose in hand, but it is 

 usually possible to determine, in the manner pointed out above, a cor- 

 rection factor to be applied to all throws, which will make the instru- 

 ment trustworthy. 



