534 



SUBMARINE CABLE LAYING AND EEPAIRING. 



causes no alteration in the deflection. The bridge is then 

 balanced and the resistance of the galvanometer is 



q= -.d units. 



The ratio arms a and h should be high, — 1,000/1,000 or 

 10,000/10,000. 



Reflecting Galvanometer as Milammeter. — ivlr. C. W. 

 Schaefer has developed the low-shunted reflecting galvanometer 

 on the potentiometer principle, making the combination to 

 cover a large range in the exact measurements of small currents 

 {The Electrician, September 5, 1902). Also by employing a 

 shunt of copper of exactly the same temperature coefficient as- 



Sullivan Reflecting 

 Galvanometer 



KDrr^- 



mX,.jL..„_3 



j_ 1 



10 4 



[Fig. 302.— Schaefer's Reflecting Milammeter. 



the galvanometer coil, all temperature variations are automatic- 

 ally compensated, provided the galvanometer and shunt are at 

 the same temperature when in use. The shunt is normally of 

 1 ohm resistance, or thereabouts, and of copper, so that it is 

 universally applicable to Sullivan and other copper-wound pro- 

 portional galvanometers. The resistance of any galvanometer 

 used must be carefully measured at the temperature at which 

 the shunt is correct. The shunt being very low in comparison 

 to the galvanometer, the resistance of the combination is prac- 

 tically constant for all ranges of sensibility. The shunt is pro- 

 vided with intermediate connections at points equal to one- 

 half, one quarter, and one tenth its resistance, as represented 

 in Fig. 302. To one or other of these points the moveable 

 galvanometer wire is connected according to the range of sensi- 

 bility suitable for the work in hand. 



