RECENT PROGRESS IN PHYSICS. 



437 



r representing the semi-diameter, I the length of the wire in the 

 globe. The specific gravity of platinum is 21, and its specific heat is 



0.0031. 



Performing the multiplication indicated, we get 



T = (0.0074 70 (l + ^.oVfi.- )■ 



According to this formula, the rise of temperature T' of the wire 

 can be computed when the corresponding depression h is observed, 

 and the dimensions of the platinum wire known. 



A wire, for which r =. 0.036 lines, 

 and Z= 59.7 *' 

 gave the following data : 



For all the respective values of s, h, q, computing the value of n in 

 the equation 



h 



q' 



we get for the mean value of n 0.91.. 



When "i^, 

 s 



r — 



Ih = 0.91, 



the corresponding rise of temperature, therefore, would be 0.91 X 

 O.0074 =: 0.006734, and the rise of temperature of the wire 



T' = 0.006734 (1 +55.24) 



T' = 0.3787. 

 Biess found for this case with his formulas, which are developed in 

 a less simple manner, at a temperature of 12°. 5, T = 0.3975, which 

 is nearly equal to the above value. 



The quotient ^i^-T\ becomes greater as the wire is finer and M C 



shorter, or as M becomes less. In most cases which present them- 

 selves in such researches M is so small, as in the above case, that the 



171 C 



fraction -rrr^ is considerably greater than 1. 



In this discussion we have supposed the temperature to be 15°. If 

 the temperature of the air had not been 15°, but 0°, the air would 

 have been denser in the proportion of 1 to 1.0547, and m would have 

 been so much greater. 



The temperature of the air being 15°, we found above that a de- 

 pression of one line corresponded to a rise of temperature of 0°.0074. 

 If we had taken 0° for the starting point we should have found that a 

 depression of one line corresponded to a rise of temperature of 

 0^.0070; if, therefore, the experiments had been made at 0°, the 



