PEIRCE. — BALLISTIC GALVANOMETERS OF LONG PERIOD. 



287 



MS 



B' 



W 



times objectionable) for a suspended system which in order to have the 

 requisite moment of inertia must weigh perhaps 300 grams. Silk in- 

 sulating material is generally magnetic, and so is most paraffine wax. 

 A certain closed frame made by Mr. G. W. Thompson, the mechanician 

 of the Jefferson Physical Laboratory, of the best obtainable sheet cop- 

 per, to hold the coil of a d'Arsonval galvanometer of the common cored 

 type, had a period of oscillation of about 2 minutes when suspended by 

 a certain piece of gimp in free space, but a period of only 9 seconds 

 when put in place in the instru- 

 ment. In this case the righting 

 moment due to the fibre was 

 clearly wholly overshadowed by 

 that due to the magnetism of 

 the copper. When copper was 

 wound on this frame, the magnetic 

 moment of the whole, if placed 

 between the poles of the perma- 

 nent magnet, became so large 

 that the whole suspended system 

 could be deflected at will, when 

 the circuit was open, by a bar 

 magnet held in the hand outside 

 the frame of the instrument. 



It is easy to make the period 

 of an ordinary d'Arsonval galvan- 

 ometer of the Ayrton and Mather 

 form as long as, say, 120 seconds, 

 by attaching two small brass 

 masses symmetrically to the ends 

 of a horizontal aluminium wire centred on the axis of suspension of 

 the coil (Figure C), though it is not always easy to balance the coils 

 and its weights so exactly that the throws shall be symmetrical on both 

 sides of the zero point and that the instrument shall not be unpleas- 

 antly affected by changes of level. Galvanometers of this kind are 

 often useful : several (one with a period of 158 seconds) have been used 

 for years in the Jefferson Laboratory, and Professor A. Zeleny has lately 

 employed a loaded coil galvanometer in his investigations of the prop- 

 erties of condensers. When the case of a d'Arsonval galvanometer is 

 large enough, it is obviously better to load the coil with a disk centred 

 on the axis of suspension than by several small masses, and in the 

 instruments to be described in this paper thin disks with strongly 

 reinforced rims were employed. 



Figure C. 



The horizontal rod AB is threaded, 

 and the brass masses C, D can be screwed 

 on the rod as far as is necessary. The 

 system must be accurately balanced. 



