256 



Prof. De la Rive on the Propagation of Electricity 

 Hydrogen. 



1 



I Pressure of the 

 gas. 



Intensity of derived current. 



Platinum disks 



near the positive 



electrode. 



Platinum disks 



near the negative 



electrode. 



millims. 



15 



6 



4 



2 



90 

 82 



52 

 35 



90 



65 



2 







It will be seen from these Tables that the intensity of the de- 

 rived current diminishes with the pressure, although the total 

 current becomes much stronger as the pressure falls, which proves 

 with what rapidity the resistance of the gas diminishes as it is 

 more and more rarefied. But at the same time the diminution 

 of the derived current, and consequently that of the resistance, 

 is much greater when the platinum disks are immersed in the 

 dark space near the negative electrode than when they are in the 

 luminous part of the electric jet near the positive electrode. 

 Thus under a pressure of 2 millims. it is not possible, in the 

 case of hydrogen, to detect the smallest derived current with 

 the disks in the dark space, whereas, when they are in the lumi- 

 nous part, the derived current still gives a deflection of 35° : 

 with 15 millims. pressure, the deflection was 90° with the disks 

 near either electrode ; but under this pressure there was not yet 

 any formation of a dark space, and consequently the condition 

 of the gas as to density was the same at both ends of the tube. 

 The resistance of the dark space is likewise very small in the 

 case of nitrogen under a pressure of 2 millims., since the deflec- 

 tion caused by the derived current is only 3°, while the current 

 derived from the luminous space gives a deflection of 18°; but 

 the difference between the two currents is less than it is with 

 hydrogen. This difference is due to the fact that, hydrogen 

 having a much greater conductivity than nitrogen, on the one 

 hand the total intensity of the discharge is greater, which is the 

 reason that we have a deflection of 35° instead of 18° in the 

 luminous space; and on the other hand, the derived portion of 

 the current must be less when the conductivity of the gas is 

 still further increased by rarefaction, in consequence of which 

 we have 0° instead of 3° in the dark space. 



We may observe that all the results which indicate an un- 

 equal resistance offered to the passage of the electric discharge 

 by different parts of the same gaseous column are perfectly 



