Measurement of some Standard Resistances. 413 



build-up contrivance is then compared with coil 2 of the 

 box, and hence with 1 a + lb. The comparisons are all done 

 on the Carey-Foster bridge, so that ultimately each coil of 

 the build-up box is known in terms of S and the bridge-wire . 

 Finally, the unknown coil X is compared with 1 a + 1 b + 2 

 + 3+4. 



Fig-. % 



L 



'a 'b s 3 4 



The objection to this method is that it takes some time, 

 and temperature changes lead to difficulty. 



Method III. — Although Method II. is applicable to boxes 

 in which the unit is greater than one ohm, it becomes difficult 

 of application unless the wire of the Carey-Foster bridge can 

 be altered. In my bridge the resistance of 1 millim. is 

 •00005 ohm, and the difference between two coils of 50 or 

 100 ohms, nominal value, may easily be greater than the 

 whole bridge-wire resistance. 



It was desirable therefore to devise a method of measuring 

 the resistance of the coils of the box which did not involve 

 the Carey-Foster bridge for the higher resistances. 



Two 10-ohm coils P and Q were taken and their difference 

 accurately found on the Carey- Foster bridge ; thus, the differ- 

 ence being small, the ratio P/Q is known accurately. The 

 value of the coil 1 a is also found on the Carey-Foster bridge. 



Four mercury cups, A, B, C, I) (fig. 2), are arranged at 

 the corner of a square ; the gaps AB, BO contain the coils P, Q 



