436 RECENT PROGRESS IN PHYSICS. 



in order to be in equilibrium with the pressure of the air ; hence we 

 can consider 44,600 as the measure of the tension of the air in the 

 globe. 



The temperature of the air being increased 1° (to 16°) it dilates in 

 the proportion : 



(1 4- 15 X 0.00365) to (1 + 16 X 0.00365), 

 1.05475 to 1.05840, 

 1 to 1.00346; 



the air of 15° consequently dilates 0.00346 of its volume for each de- 

 gree of temperature above 15°. 



But if the air cannot dilate, its tension increases in the same pro- 

 portion, hence we have 



1 : 1.00346 =: 44,600'" : 44,754. 



Thus a rise of temperature of 1° produces a depression of 154 lines 

 in the tube, provided no increase of volume takes place ; a depression of 

 1 line, therefore, corresponds to a rise of temperature of yl^ = 0°. 00649 

 when the increase of tension alone is considered. 



The capacity of the globe amounts to 320,307 units of division ot 

 the tube. A rise of temperature from 15° to 16° would expand the 

 air in the globe 1108 such units, if the air could expand freely; hence 

 an increase in volume of 1 line in the tube corresponds to a rise of 

 temperature of y-Jo^rr: 0°.000y. 



A depression of 1 line, considering the increase of tension and of 

 volume, corresponds to a temperature 



0°. 00649 + 0°.0t)09 = 0°.0074. 



From the elevation of temperature of the air in the globe, that of 

 the wire can be found. Let t be the temperature of the air and of the 

 wire before discharge ; T the temperature of the wire after the dis- 

 charge ; t' the rise of temperature which the wire causes by imparting 

 its excess of heat to the air ; then 



MC(T — = wc(^' — 0, 

 M representing the mass and C the specific heat of the platinum wire, 

 m the mass and c the specific heat of the air in the globe. From this 

 equation we get 



m / _ /./ . m c -f M C 



or 



T = (*' -0 (l + ^) = 0.0074 A (l + B^), 



T' indicating the rise of temperature of the wire, t' — i^ is the rise of 

 temperature of the air, which can be computed easily from the ob- 

 served depression. 



The capacity of the globe is 40766 cubic lines ; the specific gravity 

 of the air, at 15°, is 0.00114, the specific heat of the air 0.188 ; we 

 have, therefore, 



T' - ro 0074 h) (l 4- 40^66 x 0.00114 x 0.188 \ 



