in Terms of the Mechanical Equivalent of Heat 25 



Gc cos + GV= H sin 

 Gc cos d'-G'c = H sin 6' 



Hence c~- tan U0+6') c>=— *™ ^ d ~ 6) 



nence c- Q tan ^+o ), c Q ^ ^ + ^ 



Let Z denote the length of the wire in the fixed coil and b the 



4;r 2 

 distance of the needle from its plane. Then G= 



l{l + V7rb 2 ) 



Hence the equation for J becomes 



r J BH tan ${6 + 6') sin £(0-0') 

 J — ■ 



4tt 2 G' A cos 4(0.+:0/) 



I shall discuss in order the quantities contained in this expres- 

 sion. 



R', the resistance of the secondary circuit, is the sum of the 

 resistances of the 30,000 ohm coil, the sine galvanometer and 

 connecting wires. The whole was measured by connecting 

 the terminals of the circuit with a Jenkin bridge and comparing 

 with other coils, using a high resistance Thomson galvanometer. 

 The provisional standard was a 10 ohm coil, A, made by 

 Warden, Muirhead and Clarke. From this coil the resistance 

 of a 100 ohm standard, B, was obtained by means of a com- 

 parator, C, of 10 coils, each nearly equal to A, each coil of C 

 being compared with A, and C in series then compared with 

 B. Then A, B and C were arranged to form a bridge with D, 

 a 1,000 ohm standard, whose resistance was thus fixed. E and 

 F, two 1,000 ohm coils of a resistance box, were then com- 

 pared with D. Finally a bridge was formed with A, B, D + 

 E + F, and R', the secondary circuit, giving R' in terms of A. 

 Elliott's coils were used in making the adjustments, which were 

 always very small, and the temperatures were carefully observed. 

 The result is 



R'=300124 at 19°*3 C. 

 R' consisted principally of the 30,000 ohm coil and the vari« 

 ation of this only need to be considered. Its temperature 

 varied from 19° to 24° when in use, the mean temperature 

 being 22-°3 C. At this temperature, R' = 30052, which value 

 was used throughout. 



The length of the wire in the fixed coil was determined by 

 measuring with a steel tape the distance between two threads 

 fastened on the wire before it was placed on the circle. When 

 the wire was in position, the interval of a few centimeters 

 between the threads was measured. The tape had been com- 

 pared with standards. Care was taken to avoid difference 

 of tension in the two positions of the wire. The result is 

 Z=26449 cm . 



