THK MEASUREMENT OF CURRENT 



11 



dit ions, the result being a continual shifting of the zero point. 

 To remove this trouble, C. Vernon Boys introduced the use of 

 quartz-fiber suspension-. 



Quartz when melted is very viscous and may readily be drawn 

 into long threads of uniform cross-section. Intrinsically, quartz 

 fibers are very stiff but this is compensated for by their great 

 strength which permits the use of exceedingly fine threads. A 

 fiber 0.0014 cm. in diameter breaks under a weight of about 

 10 grn. and may be used to 

 carry 5 gm.; finer threads 

 break at even higher stresses 

 per unit area. Fibers as long 

 as 8 or 10 cm. may be cm- 

 ployed for galvanometer sus- 

 pensions.* 



It is found that with quarti 

 fibers the twist produced by a 

 given turning moment is ac- 

 curately proportional to the 

 moment and independent of 

 the previous history of the 

 thread; this very important 

 property allows quartz-fiber 

 suspensions to be used in many 

 sorts of instruments where a 

 delicate torsional control is de- 

 sired. 



The Needle System. The 

 system must be light with the 

 masses symmetrically placed 

 with respect to the axis of rotation. For this reason the shank 

 canving the needle and the mirror (be in Fig. 6) must be per- 

 fectly straight. A very slender glass or q ua rtz rod such as is 

 used for this purpose will be straightened, if, while held verti- 

 cally under the tension of a weight, it is stroked up and down 

 with a yellow gas flame. 



*The manipulation of fused quartz is discussed in THRELFALL'S book 

 " ( )n Laboratory Arts," p. 196. Quartz fibers an- now articles of commerce 

 and may be obtaim-,1 ,,f th ; Ilanovia Chemical Co., New York City. 



I 'i<;. 7. Du Bois-Rubens shielded 

 galvanometer. . 



