88 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The apparent moment of inertia of the system seems usually to. 

 remain practically unaltered when the moment of the restoring couple 

 which dominates the swings is changed within wide limits, but 

 under these circumstances the coefficient of damping generally in- 

 creases rapidly as the restoring moment is increased, and the period 

 decreases. If the restoring moment is due to an external field the 

 periodic time remains fairly constant as the amplitude decreases ; but 

 if the moment comes from the torsional rigidity of a stiff wire, the 

 period frequently lengthens somewhat as the amplitude grows small. 

 In case of a d'Arsonval galvanometer coil hung by different pieces 

 of gimp or wire successively, the damping coefficient is practically the 

 same for large differences of period if the resistance of the coil circuit 

 is unchanged; but if this resistance is changed, the damping coeffi- 

 cient changes in a manner to be quantitative!)' explained by assuming 

 that the coil has an apparent resistance larger than its real resistance. 

 This apparent resistance may be considered as a constant of the coil 

 as long as the level of the instrument is unchanged. If the righting 

 moment of a swinging coil or magnet exposed to air damping is weak 

 and comes from the torsional rigidity of a piece of fine gimp or fibre, 

 the motion often seems to be anomalous because it depends upon ob- 

 scure elastic changes. 



The Jefferson Laboratory, 

 Cambridge, Mass. 



