PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 



55 



will also be incidental expenses connected with the making and standard- 

 ising of the thermometers. For these purposes the Committee ask for a 

 grant of 75Z. 



The Committee therefore recommend that they be reappointed, with 

 a grant of 751., and that Lord Rayleigh be Chairman and Mr. E,. T. Glaze- 

 brook Secretary. 



APPENDIX. 



^ote on an Improved Standard Resistance Coil. 

 By Robert S. Whipple. 



The coil in question consists of a bare wire wound on a mica frame. 



This form of coil possesses the following advantage over the ordinary 

 resistance coil : — (1) The coils can be annealed to a dull red heat in situ, 

 thus relieving the wire of any strain caused by the winding. (2) The 

 heating of a wire immersed in oil is less than one silk-covered and 

 varnished. (3) The temperature of the wire can be accurately determined 

 by means of a thermometer placed in the oil surrounding the wire. 

 German physicists have adopted a form of coil in which the wire is 

 silk-covered and varnished and then placed in a metal case perforated 

 with holes. The whole coil is placed in an oil bath when in use. This 

 form of coil is open to the objection that it cannot be annealed above 

 140° C. without causing injury to the silk covering on the wire, and there 

 is a certain amount of lag in the oil obtaining the temperature of the 

 coil. 



By request of the Electrical Standards Department of the Board of 

 Trade the Scientific Instrument Co., Cambridge, have designed and 

 made two standard 1 -ohm coils the wires of which are bare and immersed 

 in oil ; a modification suggested by Mr. Horace Darwin was also fitted 

 for obtaining the temperature of the coils. The coils proper consist of 

 0'035 in. PtAg wire wound on mica frames, the ends of the wires being 

 attached to stout copper terminals in the usual manner. A 0'08 in. platinum 

 wire is wound alternately with the platinum-silver wire, and is attached 

 similarly to stout copper leads. Both coils are adjusted to a resistance 

 of 1 ohm at 15°'5 C. Owing to the diflei-ence in the temperature coefficient 

 of the two wires (PtAg 0-00024, Pt 0-00350), a small change in the 

 temperatui-e of the coil causes a comparatively large difierence between 

 the resistances of the two coils. This difference being known, the 

 temperatures in degrees Centigrade is given by the adjoined table. The 

 table is calculated from the difference in the temperature coefficients of 

 the two wires 0-00350-0-00024=0-00326 for 1° C. 



Temperature of 

 standard coil 

 10°-0 C. 

 lI°-0 C. 

 ll°-9 C. 

 13°0 C. 

 14°-0 C. 

 15°-0 C. 

 15°-5 C. 

 16°0 C. 

 17°-0 C. 

 18°-0 C. 

 l!)°-0 C. 

 20°-0 C. 



Difference in resistance 



of the coils 



-001793. 



0-01467 



0-01141 



0-00815 



000489 

 -0-00163 



000000 

 + 0-00163 \ 



0-00489 I 



0-00815 !- 



001141 1 

 + 001467'' 



Platinum coil having a lower 

 resistance than the platinum- 

 silver coil. 



Platinum coil having a higher 

 resistance than the platinum- 

 silver coil. 



