286 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



instruments. It seemed necessary, therefore, to increase the moment 

 of the suspended system so much that in spite of a stiff suspending 

 gimp the period should be long. 



In the case of a galvanometer coil with a period several minutes 

 long, it is difficult to tell by mere inspection for a few seconds whether 

 the coil is really at rest at its zero or whether it has a very slight velocity 



Figure B. 

 This magnet has a solid core which weighs ahout 1500 kilograms. 



which in the course of its slow swing will lead to an addition of two or 

 three millimeters to the amplitude of the throw. For this reason it was 

 desirable that the coil should be subjected to some slight electromag- 

 netic damping, though, as will appear later on, it was not possible to 

 damp the coil critically. 



The requirements enumerated above are so simple that it seemed at 

 first that there would be no difficulty in meeting them all, and this 

 would have been the case if it were not for the fact that the best cop- 

 per and silver wire, and the best copper, silver, and aluminium sheet to 

 be had in the market are usually so highly paramagnetic that in an 

 intense magnetic field the galvanometer coil and the metal frame on 

 which it is wound, if a frame be used, often acquire a large magnetic 

 moment, and this increases in an irregular way the righting moment of 

 the suspended system — perhaps to many times the value due to the 

 gimp alone. The difficulty is an old one ; many persons have struggled 

 with it, and some have succeeded in overcoming it more or less com- 

 pletely, by great care in the preparation of special wire for the purpose. 

 The difficulties are, however, very much increased when it is necessary 

 to provide a sufficient electromagnetic damping (air damping is some- 



