PEIRCE. 



BALLISTIC GALVANOMETERS OF LONG PERIOD. 



301 



a fairly strong damping what the value of the logarithmic decrement 

 should be. 



A certain d'Arsonval galvanometer (X), of the type represented in 

 Figure C, which was formerly in use in the Jefferson Laboratory, had a 

 period of 149 seconds. When the coil was given a deflection corre- 

 sponding to a scale reading of 14.15 cms., and was then allowed to 

 swing, the ratios of the successive half amplitudes were 1.066, 1.061, 

 1.067, 1.061, 1.066, 1.060, etc. 



TABLE I. 



The galvanometers (X, Y) j ust mentioned, unlike most of those which 

 are usually available in a laboratory, were almost exactly symmetrical 

 in their throws on opposite sides of the zero. In most large instruments 

 in which the coils are wound on open metal frames, there is a slight 

 •bias, so that a given flow of electricity sent impulsively through the 

 circuit causes a little larger throw on one side than on the other. 

 Sometimes the bias, when the always small throw is increased by in- 

 creasing the discharge, changes sign ; sometimes levelling the instru- 

 ment will help a trifle, but usually the lack of symmetry seems to be 

 connected with the magnetism induced in the frame or the coil by the 

 field of the magnet. 



Mr. John Coulson, who has studied in the Jefferson Laboratory the 

 characteristics of an excellent short period d'Arsonval galvanometer of 

 the very best make, has found a bias of about 2 per cent in favor of the 

 throws on one side of the zero point. In this instrument there is also the 

 same irregularity in the ratios of successive amplitudes which has been 

 already noticed. For a given impulse, which caused a throw on one 

 side, after which the coil oscillated with decreasing amplitude, the ratios 

 were 2.16, 2.03, 2.15, 2.08, while the same impulse reversed in direction 

 gave the ratios 2.09, 2.12, 2.09, 1.97. These values were persistent and 

 could be obtained over and over, and their differences were quite large 



