Prof. E. Edlund's Investigation of the Electric Light. 115 



the galvanometer amounted to 62° 6'; and when 1 3 units were 

 interposed, 37° 44'. From this are obtained M' = 8*425, and 

 E= 15-91. When 0*6 inch of the rheostat was in the circuit, 

 the following result was obtained : — 



Length of the arc = 2 divisions. Resistance =8*9; 



and for the same intensity, 



Length of the arc =1 division. Resistance =8*1. 



From experiment VIII. we get D = 0*279 x E = 6*34, and 

 from Exp. IX. D = 0*407 x E' =6*48. This series leads, there- 

 fore, to the same result as Series 2, namely that D is indepen- 

 dent of the electromotive force of the battery. 



Fifth Series of Experiments. 



8. The magnitude of the development of heat in the luminous 

 arc depends upon the intensity of the current and the resistance 

 of the arc, and is proportional to the latter. Now, since the 

 actual resistance is not greater than the preceding experiments 

 indicate, and it only amounts to a small fraction of that which 

 has hitherto been assumed, it might perhaps be doubted whether 

 this resistance is great enough to explain the formation of the 

 high temperature which takes place in the luminous arc. The 

 following experiments were undertaken with the view of investi- 

 gating this. 



The platinum wire used in my previous experiments on gal- 

 vanic expansion (1150 millims. in length and 0*542 in diameter) 

 indicated a constant expansion of 138 divisions when a current 

 passed through it whose intensity was = Tg. 43° 21'. In the 

 first second after the cessation of the current the wire cooled 

 so that it shortened by 12*94 divisions (almost exactly corre- 

 sponding to 7° of temperature). Since a current of the strength 

 in question was sufficient to keep the wire at a constant length, 

 this current must have produced an increase in temperature in 

 the wire of 7 degrees in each second, which corresponds to a de- 

 velopment of 1*27 thermal unit, if the gramme is taken as unit 

 of weight. The resistance of this wire was now measured with the 

 rheostat used in Experiments VIII. and IX. ; and it was found that 

 it amounted to 0*35 unit of resistance — something less, therefore, 

 than that of a luminous arc of the length of one division of 

 the scale (0*4 millim.) in Experiment VIII. In this experiment, 

 in which the intensity was considerably greater than in the experi- 

 ment on galvanic expansion, more than 1*27 thermal unit was 

 developed in each division of the luminous arc in one second. 

 This quantity of heat (1*27 unit) is sufficient to raise the tempe- 



12 



