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



and convection currents o£ air in the thermometer tube 

 increases nearly in proportion to the absolute temperature. 

 The effect of radiation also becomes important at high tem- 

 peratures, and the cooling is then more rapid. If, therefore, 

 the watts are kept constant, the heating effect will diminish 

 as the temperature rises, and a small systematic error will 

 be produced. Assuming that the rate of cooling increases 

 as the absolute temperature 0, and that the watts are kept 

 constant, the heating effect at any temperature is 273 h/0, 

 where h is the heating effect in degrees of temperature at 

 0° C." Prof. Callendar states that a better rule is to keep 

 the current through the thermometer constant, as in this 

 case the heating effect also is nearly constant if the current 

 flows sufficiently long for the heating effect to be steady. 



Recently, Waidner and Burgess * have measured the heat- 

 ing effect of the measuring current on a thermometer of 

 1*34 ohms fundamental interval, and found the increments 

 of temperature at 3 and 100° 0. to be 1°'65 and 1°'69 

 respectively when a current of O'l ampere was used. At 

 the sulphur point the increase of temperature for the same 

 current was 1°'20 C. 



While Ave believe it is now the general practice to keep 

 the current constant, as suggested by Prof. Callendar, we 

 thought it would be of interest to make fairly accurate 

 observations with a thermometer of 1 ohm fundamental 

 interval. The results which follow show that in our experi- 

 ments, with a current of O'l ampere, the corrections for the 

 increases of temperature at the ice and steam points are 

 practically the same as those observed by Waidner and 

 Burgess, but at the sulphur point the correction for the rise 

 of temperature is about o, 65 G. greater. In all cases the 

 diameter of the platinum wire was 15 mm. In measure- 

 ments of the highest precision it appears to us to be necessary 

 to determine the corrections for increments of temperature 

 for various currents, at temperatures over the range through 

 which it is proposed to work. 



To measure the current we placed two sensitive and similar 

 ammeters in the P and C arms of a Callendar- Griffiths 

 bridge, and thus directly measured the current through the 

 platinum thermometer. The results at 0°, 100°, and at the 

 sulphur point are given in the following tables. The correc- 

 tions given in the last columns of the tables are those to be 

 applied for the heating effect in all four arms of the bridge. 

 The current values are correct within less than 1 per cent. 



* C. W. Waidner & Burgess, Bull. Bur. of Stands, vol. vi. no. 2 

 (1910). 



