300 PROCEEDINGS OF THE AMERICAN ACADEMY. 



needle swinging in a strong field under the damping action of a mica 

 vane of the usual kind usually diminishes as the amplitudes grow 

 smaller. The same tendency often shows itself in the case of a d'Ar- 

 sonval galvanometer when the damping, either electromagnetic or 

 atmospheric, is fairly large. 



In a galvanometer of any of the common forms in which the restoring 

 moment is due, not to the mutual action of a magnet and the external 

 field, but to torsional forces in a spring or suspending fibre, even though 

 the system comes to rest sensibly at its old position of equilibrium, the 

 swings are often one-sided in a fashion best described, perhaps, with 

 the help of an example or two. 



A certain d'Arsonval galvanometer (Y) of the Ayrton and Mather 

 type was connected in series with a rheostat of resistance R and the 

 coil of a small magneto-inductor. The period of the galvanometer coil 

 was dependent of course upon the value of R : when the circuit was 

 broken, its value was about 16.5 seconds. The same flux change in 

 the coil of the inductor might be made over and over again at pleasure 

 by slipping the coil in one direction or the other between two fixed stops. 

 The resistance of the galvanometer and the inductor coil together 

 was about 96.6 ohms. When the galvanometer coil was at rest in 

 its position of equilibrium (scale reading 711), and the value of R was 

 600 ohms, the inductor coil was moved quickly from one stop to the 

 other and a short series of turning points, 329, 886, 623, 750, 689, were 

 observed. When the inductor coil was slipped back to its original place, 

 the readings were 1095, 534, 799, 672, 733. Using the first set of 

 turning points and the zero 711, the successive half amplitudes were 

 382, 175, 88, 39, 22, and the ratios of the successive pairs were 2.18, 

 1.99, 2.26, 1.77. The other set of turning points give the half ampli- 

 tudes 384, 177, 88, 39, 22, and the ratios, 217, 2.01, 2.26, 177. The 

 half sums of corresponding numbers in the two observed sets are 712, 

 710, 711, 711, 711, and there is no obvious bias in favor of deflections 

 on one side of the zero point. There was no sensible "set " when the 

 system came to rest, but during the swings there seemed to be a very 

 slight movement of the zero point towards the side of the first excur- 

 sion, at the end of which the whole angle of twist in the long gimp was 

 only about 1°. When R was made 400 ohms, the time of swing fell from 

 8.6 seconds to 8.2 seconds, the throw due to the same movement of 

 the inductor coil rose to 483, and the ratios of successive pairs of half 

 amplitudes became 3.16, 2.68, 3.17. When the twist in the gimp per 

 centimeter of its length is made as large as in many of the instruments 

 in common use, the tendency here noted becomes very troublesome, and 

 it is difficult to determine from a short set of throws corresponding to 



