72 



Messrs. Ayrton, Mather, and Sumpner 



ends of the wires W project through holes H in the brass 

 case, these holes being kept closed by tightly-fitting ebonite 

 collars C, rig. 1, E, fig. 6, when the galvanometer is not in 

 use, or when it is being employed for some test where a small 

 amount of leakage is not important. 



Still higher insulation is obtained with the galvanometer 

 shown in fig. 7, since in this instrument the coils are sup- 

 ported from two corrugated ebonite rods which hang from 

 a brass ring R, carried on the top of three corrugated ebonite 



Fur. 7. 



Very high resistance galvanometer with brass case removed. 



pillars fixed to the slate base-plate. This instrument, the four 

 coils of which have a joint resistance of nearly 400,000 ohms, 

 was constructed by Messrs. Nalder Bros.; but the device, by 

 means of which the shortest path along which surface-leakage 

 can take place from the coils, or from the terminals, to the 

 base of the instrument is between 30 and 40 inches of ebonite 



