of Electricity through Rarefied Air. 11 



elasticity modified by the passage of the current. Now, when 

 the lever is removed, the thin platinum wire is lifted from the 

 mercury, and the current is then interrupted in successive 

 experiments with such uniformity that the currents which pass 

 through the rarefied air give deflections of equal magnitude 

 on the magnetometer. 



Experiment 2. The battery that produced the inducing 

 current consisted of six Bunsen elements. The glass tube 

 containing the rarefied air was the same as in experiment 1, 

 with aluminium electrodes placed 5 millim. distant from one 

 another. The passage of the induced current through the 

 rarefied air gave the following deflections: — 



Pressures of the gas, Deflections. 



m millimetres. 



89-8 25-8 



79-6 28-0 



65-7 ..... 29-6 



58-6 33-0 



39-0 38-8 



31-1 41-7 



18-9 35-0 



8-2 3O0 



2*9 19-1 



0-58 15-9 



0-12, .... 0*0 



This series resembles those repeatedly obtained before by 

 other physicists : at a comparatively high pressure of the air 

 the current is weak ; it reaches a maximum at a certain lower 

 pressure, afterwards diminishing again if the rarefaction is 

 increased beyond this. It is from such series of experiments 

 that it has been inferred, inter alia, that the electric resistance 

 of air diminishes with the pressure until it arrives at a certain 

 limit, which being passed, the resistance increases again, if 

 the rarefaction continues to increase, and in a vacuum becomes 

 infinite. Here I will only remark, as a preliminary, that the 

 series of deflections of experiment 2 may be regarded on the 

 whole as a sort of inversion of the series of experiment 1. In 

 the latter series the deflections are considerable at the higher 

 pressures of the air, and then diminish till the pressure has 

 fallen to about 30 millim., after which they commence to in- 

 crease if the increase of rarefaction be continued ; in series 2 

 the reverse is presented, in that the maximum occurs there at 

 a pressure of about 30 millim. 



If E denotes the electromotive force of the induction -appa- 

 ratus, e that of the spark in the rarefied air, r t l f as before, the 



