﻿Connective Cooling in Fluids. 931 



with mercury cups. A separate test showed that the resist- 

 ance of the coils was independent of the current. Values 

 of dR down to O'OOl ohm were obtained by shunting a 

 resistance of 1 ohm by 1000 ohms, etc. These values were cali- 

 brated directly on a Cambridge resistance bridge, and agreed 

 with calculation. Using a current of 0*01 ampere through 

 the biidge it was possible to obtain the initial balance correct 

 to O'OOOl ohm, or rather less, an amount associated with a 

 galvanometer deflexion of 2/5 mm. on reversal of battery. 

 Thus, with a posible error of setting of 0*0001 ohm, values 

 of ^R = 0*1 would be correct to 1/10 of one per cent., and 

 generally the error would be negligible, but with the 

 smallest values of dH used (0*001 ohm) the error might be 

 between 5 and 10 per cent., and averages were taken. With 

 the heating current, of course, the bridge was much more 

 sensitive. With the 3-mil wire and a current of J amp. a 

 resistance change of O'OOOl ohm in the third arm would 

 cause a galvanometer deflexion of 1 cm. Incidentally it 

 was noticed that a change of 1 per cent, in this current 

 value caused a deflexion of 13 cm., and this was also 

 approximately the case with the larger wire. 



As a further check on the apparatus, the cooling in gases 

 was studied by a volt-drop method instead of using the 

 Wheatstone bridge arrangement. Various experimenters 

 have used this method, measuring the current through a 

 given length of wire and the volt drop along it. To mini- 

 mize end effects, experimenters have usually used potential 

 leads of very fine wire, attached at sufficient distances from 

 the heavy current leads to eliminate the cooling effects of 

 these latter. This, however, does not eliminate whatever 

 cooling is due to the fine potential leads, and the volt-drop 

 method used in the present experiments is superior in this 

 respect. The method was to pass the current though TT 

 and CO in series, and to measure the respective potential 

 falls by means of a potentiometer. The difference gave the 

 potential drop along the uncompensated length, the end 

 effects being eliminated completely. The current was 

 obtained by passing it also through a standard manganin 

 resistance and determining the volt drop for this also. The 

 resistance TT— CC at the temperature of the air was obtained 

 from the potential drop when a very small current 0*01 amp. 

 was passing. The results obtained by this check method 

 were within 1 or 2 per cent, of those obtained by means 

 of the Wheatstone bridge. It is not so convenient a 

 method for ths liquids, owing to the zero changes which are 

 easily dealt with by the Wheatstone bridge method. 



