THE MEASUREMENT OF CURRENT 



23 



N 



I 



N 



N 



The window, /b, allows the mirror to be observed, the hard 

 rubber base, BI, upon which the shields rest being cut away for 

 that purpose, as shown in the cross-section. 



The needle system is suspended by a quartz fiber, which is 

 mounted on a swinging support at p, by which the system may be 

 centered. When the coil support is opened, the needle system 

 may be swung forward for examination. 



The arched piece L carries the control magnet M, which is 

 made of bent clock spring. By it, the zero point and the sen- 

 sitiveness of the galvanometer may be changed. 

 The needle system is not adjusted for astaticism. 



The coils are wound in three sections, the inner 

 consisting of 81 cm. of No. 38 wire, the middle 

 section of 328 cm. of No. 32 wire and the outer 

 section of 1,318 cm. of No. 26 wire. Each of the 

 finished coils has a resistance of 5.6 ohms. 



To guard against the effects of static charges, 

 the plane faces of the coils are covered with tin 

 foil. The terminal wires of each coil are twisted 

 together and carried down through channels in 

 the upright and each coil has its own set of ter- 

 minals. 



The coils are mounted in the support by means 

 of an insulating wax. The shields are those re- 

 ferred to on page 9. 



The sensitivity attained with all the coils in parallel is 3 X 10 9 , 

 the time of a complete swing being 6 seconds. 



The Broca Galvanometer. 6 Various experimenters have 

 suggested the use of a needle system consisting of two slender 

 vertical magnets as shown at A in Fig. 11. 



In this case two pairs of coils would be used, acting respectively 

 on the upper and lower pair of poles. Practically, it is very diffi- 

 cult, if not impossible, to astaticize such a system, for the magnets 

 must be exactly parallel to the axis of rotation. 



In the Broca instrument the vertical needles are magnetized 

 with consequent poles at the middle of their lengths as shown at 

 B in Fig. 11. Hence it is easy to astaticize the system by re- 

 touching the magnets, and this is its advantage. 



N S 



A 



FIG. 

 Movable 

 tern with ver- 

 tical needles. 



B 

 11. 



sys- 



