722 



Dr. J. S. Gr. Thomas on a Null-Deflexion 



the temperature of the surroundings of the enclosure con- 

 taining the wire was 0°0. An investigation of the depen- 

 dence of the temperature of a fine platinum wire heated 

 by a constant current within an enclosure, upon the 

 surrounding temperature, showed that, in the present case, 

 no appreciable error would be introduced by assuming that 

 an increase in the temperature of the medium surrounding 

 the enclosure containing the wire, is accompanied by an 

 increase in the temperature of the heated wire equal to such 

 increase of temperature. The results plotted in fig. 2 have 

 thus been deduced from the experimental results, assuming 

 that the enclosure is surrounded by a medium at 0° C The 

 values of the resistance and temperature of the wire in the 

 horizontal and vertical positions when heated by various 

 currents, the surroundings being at 0° C, are set out in 

 Table I. herewith : — 



Table I. 





Resistance of Wire 

 (ohm). 



Mean temperature of 

 Wire (° 0.). 



Ratio =r— • 

 Ra 





Heating 

 Current 



(amp.). 



Wire 



Horizontal. 



Ra. 



Wire 



Vertical. 



Ry. 



Wire 



Horizontal. 



Qh. 



Wire 



Vertical. 



VV. 



9v-9 h 



(°G| 



0-9 



0-4189 



0-4389 



225 



250 



1-048 



25 



10 



0-4680 



0-4950 



288 



322 



1-058 



34 



1.1 



0-5273 



0-5637 



364 



414 



1-069 



50 



1-2 



0-5958 



0-6350 



456 



510 



1066 



54 



1-3 



0-6692 



0-7154 



557 



622 



1-069 



65 



14 



0-7428 



0-7936 



663 



738 



1-068 



75 



15 



0-8260 



0-8680 



786 



852 



1-050 



66 



It will be seen that on rotation of the wjire from the 

 horizontal to the vertical position, an increase of from 5 to 

 6 per cent, occurs in the resistance of the wire. The 

 accompanying increase in the temperature of the wire is 

 shown in the 7th column. The temperatures h and V are 

 probably correct to within 0'5 per cent. 



Employing the unshielded wire in the manner already 

 described, a series of determinations was made of the angular 

 rotation of the wire about the axis of the flow tube necessary 

 in order to maintain a balanced condition of the bridge when 

 the wire was subjected to the cooling action of a current of 

 air moving with a determinable mean velocity in the tube, 



