Electric Discharge in Gases, 39 



mena occur when the machine is made to work very slowly, 

 and consequently the supply of electricity is much reduced. 

 If the pressure is so low that the dark negative space is about 

 2 centim. in breadth, whilst the glow-rays are separated from the 

 positive discharge by the dividing space, or even if there is no 

 positive discharge to be seen in the tube, and if then the 

 machine is worked more and more slowly, the positive light 

 expands round the positive electrode, whilst the negative light 

 also expands considerably, until the positive and negative 

 luminosities meet. This condition may be maintained for any 

 length of time. 



Since the experiments described above had shown that the 

 discharges obtained simply from an electrophoric machine 

 represent the simplest mode of discharge, it was necessary to 

 investigate whether the propositions first stated by Gr. Wiede- 

 mann and R. Riihlmann * and myself f as to the independence 

 of the heating of the gases upon the width of the tube, and the 

 quantity of electricity traversing it, would also hold good for 

 the case in which, by removing resistances, the quantity of 

 electricity passing in each discharge is increased. If it is 

 shown that changing the resistances produces no considerable 

 effect upon the quantity of heat produced either in a wide or 

 in a narrow tube, then it follows that if these two tubes are 

 included in the same circuit, equal quantities of heat must be 

 produced per unit of length, independent of the cross-section 

 of the tube. 



The experiments are arranged exactly like those described 

 in a previous communication J. In these experiments the 

 whole quantity of heat produced and that evolved within the 

 space connecting the electrodes (in the case of spectral tubes 

 within the capillary part) were determined. Experiments on 

 the heating of the electrodes themselves will follow later. 



Letp denote the pressure, mW denote with included resist- 

 ances, oW without resistances, F that no resistance but a 

 spark 5 millim. long was included ; + or — that the positive 

 or negative electrode of the machine, as the case may be, was 

 connected with the tube, that both were connected with the 

 machine ; x as before denotes a very low pressure, and x x a 

 still lower pressure. The numbers under +, — , are num- 

 bers which are proportional to the quantities of heat evolved 

 in the unit time by equal currents. They represent here only 

 the relative , not the absolute values. 



* Pogg. Ann. cxlviii. pp. 35, 252 (1876V 



t Wied. Ann. x. p. 202 (1880); Phil. Mag. vol. x. p. 357. 



t Ibid. 



