Bare Wire for Resistance-coils. 217 



approximate 3 ohm coils *, first placed in parallel and 

 balanced against a 1 ohm coil, and then in series with the 

 1 ohm coil and balanced against the 10 ohm coil. Instead 

 of connecting the legs of the coils by mercury-cups, as is 

 usually done, the fusible metal made by Mr. F. Thomas, of 

 Cambridge, was used. This metal, which melts at 60°, gives 

 very constant contact resistances, and does not seem to injure 

 in any way the amalgamated terminals of the coils. I first 

 determined the value of a manganin 10 ohm coil, which had 

 been made for nearly a year. The value obtained on 

 December 20, 1895, was 9'9987 at 9°'3. This coil was used 

 as a standard to which to adjust the platinum-silver 10 ohm 

 coil. The value thus obtained for the latter was 9'9895 at 

 10 o, 3. On stepping up to the platinum-silver 10 ohm coil, 

 the value found on December 30 was 9-9905 at 10°*6. 

 Taking into account the known temperature-coefficient, the 

 latter at 10 o, 3 would be 9*9897, which differs by only two 

 parts in 100,000 from its former value. 



To obtain the value of the -^ ohm coil, which was of man- 

 ganin, a bridge was form; d of two 1 ohm coils, a 10 ohm 

 coil, and the j 1 ^ ohm coil, and a balance obtained by shunting 

 one of the 1 ohm coils f. 



On January 12th, a value of '099934 at 11°*2 was obtained. 

 „ „ '13th, „ ,,-099932 „10°:2 „ „ 



The, temperature-coefficient of the wire was known to be 

 •000004 per degree. Between these two determinations the 

 bridge had been dismounted. This coil was again tested on 

 April 8th, and had then a value of '099944 at 12°'l, whereas, 

 if it had remained unaltered, its value at that temperature 

 should have been "099938. This change is, however, too 

 small to cause any error in the box-coils. 



For testing coils which are very nearly in the proportion 

 of 1 to any power of 10 this method is extremely sensitive, 

 and certainly more accurate than making the comparison by 

 means of the wire bridge, provided that the resistances of 

 three out of the four coils forming the bridge are accurately 

 known. 



In obtaining the values of the 1 ohm and 10 ohm standard 

 coils, not more than 50 scale-divisions of the wire had been 

 employed, hence a small error in the total resistance of the 

 bridge-wire would not cause any inaccuracy in the values so 

 obtained. The resistance of the bridge-wire was determined 



* This method is described ill the B. A. Eepoits, 1883. 



t This method is described by Mr. Glazebrook, 13. A. Eeports, 1894, 



