70 



Messrs. Ayrton, Mather, and Sumpner 



galvanometer for measuring currents by direct comparison 

 with the ammeter. Its absolute calibration as a ballistic gal- 

 vanometer can then be determined from the fact that 



The swing per micro-coulomb in scale-divisions 



= the deflexion per micro-ampere x -^- y 



where T is the periodic time of oscillation of the needle in 

 seconds. 



IV. Supporting the Coils. 



In olden days Messrs. Elliott supported the coils in their 

 galvanometers by simply employing three long screws to 

 squeeze each coil between a brass plate and the framework of 

 the instrument to which the plate was screwed. The disad- 

 vantage, however, of such an arrangement was made painfully 

 clear in the use of a galvanometer constructed by this firm 

 for one of the authors in 1 868. For on transporting this gal- 

 vanometer across India, soon after it was made, the brass screws 

 expanded more than the coils, and the coils consequently slipped 

 down, shearing away the needles, the mirror, and the alumi- 

 nium vane. It was therefore suggested to Messrs. Elliott that 



Fipr. 6. 



the coils should be supported in boxes B, preferably hinged, 

 as shown in fig. 6, so that when the coil-boxes are opened, as 

 in this figure, the suspended system S could be got at and 



