438 RECENT PROGRESS IN PHYSICS. 



factor 0.0070 would have been substituted for 0.0074, or a factor 

 which is less in the proportion of 1 to 1.0571 ; on the contrary, m, 



the value of the expression in the brackets, (since 1 to jr^ is very 



small,) would become greater in the proportion of 1 : 1.0547. Thus 

 the one factor would increase in almost exactly the same proportion 

 in which the other decreased, and the value of T' would remain 

 almost without change ; hence it follows that slight fluctuations in 

 the temperature of the surrounding air may be totally disregarded, 

 and no correction of the value of T' computed for 15° is necessary. 



A similar discussion of the height of the barometer leads to the 

 same result — that is to say, although our formula is computed for a 

 height of 336 lines, it may be used for other heights, because the in- 

 termediate fluctuations of the barometer have no marked influence on 

 the value of T'. 



§ 44. Influence of the length op the connecting wire on its rise of 

 TEMPERATURE. — We have seen above that, when the same discharge 

 passes through a series of wires introduced into the circuit together, 

 the heating of the separate pieces is, independent of their length, 

 and inversely proportional to the fourth power of their semi-diameters. 



But as soon as tlie circuit is considerably prolonged, by the intro- 

 duction of new wires, the heat in all parts of the circuit decreases 



In order to investigate the influence of an increase of length in the 

 circuit, Riess interposed, in succession, pieces of the same copper wire 

 of different lengths, by means of Henley's discharger, retaining in the 

 thermometer the same platinum wire. With each piece an experi- 

 mental series of the same kind was made, as shown on page 426. 

 Indicating the length of the interposed copper wire (its thickness 

 being 0.29 lines) by X. 



For^r^O , A =0.78-^ 



s 



s 



s 

 98.4"., h=: 0.34 -i- 



A = 49.0"., h=z 0.48 



5 



" ;= 147.7"., 7t= 0.27 i- 



'' A = 240.4"., A = 0.21 -21 



s 

 We see from these data that the heating constantly decreases as 



the wires increase in length, the value of ^~ being constant. 



The values of h are evidently proportional to the co-efficients of 



^~. For ^- = 1 we have the following relation between h and I: 



8 S 



J, _ 0.78 ... 



''— r+xoisi ^ -' 



