Adjustment of Resistance- Coils. 267 



the final value. It is required to find the value S of a shunt- 

 coil which will reduce its resistance to the final exact value R. 

 By the ordinary rule for divided circuits we have 



R(3? + S) = 9?S; 

 whence 



9?R 



ffi-K" 



:S. 



4. An example for the adjustment of a 1-ohm coil will 

 illustrate the method and its advantages. The following data 

 are taken from the note-book of Mr. C. C. Starling, Demon- 

 strator in the Physical Laboratory of University College, 

 Bristol, January 30, 1884. Coil marked " <£ 3." 



1*1 metre of German-silver wire (No. 24 B.W.G.), diam. 

 0-56 inillim., cut off and soldered to terminals. Tested (by 

 Foster's method) against Cambridge Unit No. 6. 



« = 1-0385518 (temp. 17°-2 C.) 



1-0385518 QQ , . , , 



————-—- ={3o'y onms required as shunt. 

 •038o518 ^ 



7*05 metres of German-silver wire (No. 36 B.W.G.), diam. 

 0*2 millim. (at 20*8 centim. per ohm), cut off, and without 

 further measurement soldered to terminals. Immediately 

 tested by Foster's method, 



R= 1-007912 Rayleigh ohms (temp. 17°-3 C). 



Time required to make and test coil and shunt 38 minutes. 



This coil was tested again three days later, namely on 

 February 3, and gave 



R= 1-004178 (temp. 13°-5 C). 



Tested again, February 5, 



R= 1-0050854 (temp. 14°-5 C). 



Tested again, February 21, 



R= 1-0063923 (temp. 15°*5 C). 



This coil is therefore correct at 0°*97 C. 



One other example of the application of the method to a bad 

 case will suffice. In this instance the resistance of the main 

 wire was not near enough to the final value, being more than 

 14 per cent, too great, and the shunt consequently shorter 

 than desirable. Nevertheless the result was quite satisfactory. 



T2 



