Form of Resistance-Balance. 



113 



positions all along the bridge-wire, and found to be so nearly 

 the same that no Table of calibration was deemed requisite. 

 To protect the bridge- wire from injuries, as well as to preserve 

 it from being heated by radiation from surrounding bodies, a 

 wooden ring, vv, is fastened down on the base-board. The 

 ring is 1£ inch wide and f inch deep; and its internal dia- 

 meter is 1 inch greater than that of the ebonite disk. The 

 wire, therefore, lies hidden away on the side of a square-sec- 

 tioned circular tube ; and, furthermore, a shield of cardboard 

 faced with tinfoil lies upon the face of the disk e, extending 

 just beyond the ring. An aperture is cut in this shield to 

 permit the passage of the trigger, as well as to allow the 

 vernier to be read. By this means the wire is not only out of 

 sight, but out of reach of all radiation as well as mechanical 

 injury. 



4. Method of determining the Variation-coefficients of Coils. — 

 To determine the variation -coefficient of any given coil we 

 proceed as follows: — Three other coils are provided, two of 

 them nearly equal in resistance, which we will call 1 and 2. 

 A third coil, 3, must be taken whose resistance is nearly equal 

 to that of 4, the coil whose variation-coefficient is desired (see 

 fig. 5). The terminals of 3 are inserted in the mercury-cups 

 a and c, those of 4 in V and df 9 those of 1 in p and r, and 

 those of 2 in q r and /. Now the operation to be conducted is 

 to keep the coils 1, 2, and 3 at a fixed temperature, and to 

 keep 4 successively at two known temperatures, differing by 

 about 15° Cent., and to obtain the difference of the resistances 

 of 3 and 4 at these two temperatures. The difference of these 

 differences, divided by the difference of the temperatures, is 

 the mean coefficient of variation of resistance between these 

 temperatures. The chief difficulty to be contended with is 

 that of keeping the temperature of the coils constant during 

 the operation, and of ascertaining what that temperature is; 



Phil. Mag. S. 5. VoL 9. No. 54. Feb. 1880. I 



