of Electricity through Rarefied Air. 19 



Experiment 7. Two perfectly similar glass tubes, provided 

 at their extremities with electrodes consisting of platinum 

 wires, were connected with the mercury air-pump so that 

 there might be equal pressure of air in both. The electrodes 

 were covered with glass tubes to the distance of 1 millim. 

 from their extremities. The distance between the electrodes 

 amounted to 5 millim. in one of the tubes, and six times that 

 sum, or 30 millim., in the other. Although the electrodes had 

 been rendered as equal as possible, from some unknown cause 

 the electromotive force of the tube in which the distance be- 

 tween the electrodes amounted to 30 millim. was greater than 

 in the other. With these two tubes the following deflections, 

 at different pressures of the air, were obtained : — 



Pressure of 



Deflections. 



the air. Distance Distance 



millim. 30 millim. 5 millim. 



0-23 .... 5-6 2-6 



0-73 .... 17-8 14-5 



1-15 .... 16-9 16-1 



2-13 .... 5-5 9-1 



5-40 .... 3-2 5-8 



8-13 .... 1-6 2-2 



Thus, at the lower pressures from 0*23 to 1*15 millim. the 

 current in the tube in which the distance between the elec- 

 trodes amounted to 30 millim. was stronger than that of the 

 other tube ; while the opposite took place from the last-men- 

 tioned pressure to that of 8*13 millim. If we call the electro- 

 motive force of the tube with 5 millim. distance between the 

 electrodes e, and that of the other tube <? 1? and if r-J, denotes 

 the resistance of the gas in the first tube, and consequently 

 6r x l the force of resistance of the second, we get, according to 



e e 



Ohm's law, — 7 and y—. The observations above given show 



nl W _ ex e 



that at a pressure of 0*73 millim., for example, -x — 7 > , 7 « If 



or^l r±t 



we pass thence to a greater pressure, as, for instance, 5 '4 mil- 

 lim., ei and e will be modified in the same proportion, and 

 consequently their amount at that pressure will be expressed 

 by pe x and pe,\\\ which case, from what precedes, p is less than 

 unity. The resistances change in the same manner into nftrj 

 and nr x l, where n denotes a number higher than unity. We 



get, therefore, at the higher pressure, -——, and ^-— 1 ; but, 



whatever numbers p and n may represent, the former fraction 



C2 



