552 Mr. F. E. Smith on Bridge Methods for Resistance 



resistance, without any reduction of sensitiveness due to a 

 smaller upper limiting value for the current *. 



In Methods 3 and 4 described in this paper, high resistance- 

 coils of manganin are so placed in series with a platinum 

 thermometer, but these do not decrease the sensitiveness of 

 the bridge (in fact they slightly increase the sensitiveness), 

 and the increase of resistance of these coils due to the current 

 passing through them is, in general, negligible. 



To remove the possibility of any appreciable error being 

 introduced due to the heating effect throughout the bridge, 

 and to simplify the labour of calculation, it is best to experi- 

 mentally determine the correction to be applied to a thermo- 

 meter reading in the way that has been done in Tables IV. 

 to VI. The corrections given in the last columns of these 

 tables do not give the temperature rise of the thermometer 

 due to the current, but the correction to the thermometer- 

 reading for the heating in all four arms of the bridge. 

 However, in the experiments cited, the heating of the 

 manganin coils was so small that the corrections are practically 

 equal to the temperature rise of the thermometer. The 

 observations on which the corrections are based occupied 

 about 30 minutes for each of the fixed points. 



We would further suggest that the coils of a thermometer 

 bridge be immersed in well-stirred paraffin oil kept at a 

 constant temperature, as such a procedure considerably 

 increases the precision of the measurements. When standard- 

 ising the bridge the resistance of the coils should be calculated 

 for an infinitely small measuring current. 



Resistance Measuring Apparatus. 



We now proceed to describe in detail several new methods 

 for measuring the resistance of a platinum thermometer, all 

 of which practically eliminate the effects of small changes in 

 the relative resistance of the leads, and have, we believe, 

 several advantages over methods hitherto used. 



In the most refined work in platinum thermometry 

 measurements are made within 0°*001 C, and we have, 

 therefore, adopted this as a standard by which methods of 

 measurement may be compared. Also we have designed the 

 resistance-bridges so that thermometers with current and 



* In 1893 Prof. C alien dar employed platinum silver coils in series 

 with a platinum thermometer, the resistance and dimensions of the coils 

 being - chosen so that the percentage increase of resistance of the platinum 

 thermometer due to the current was practically equal to the percentage 

 increase of resistance of the platinum silver. Phil. Trans. A. cxcix. p. 55 

 (1902). 



