Electricity in Rarefied Air. 127 
accompanying figure, 300 millim. long and of 16 millim. 
external diameter, is provided 
with two platinum wires, a and 
b, the ends of which were only 
3 millim. apart, and attached 
by its small end, fe, to the 
mercury-pump ; ¢ and d are two pieces of tinfoil which 
surround the tube without being in communication with the 
electrodes. 
The current was produced by a large Ruhmkorff’s induc- 
tion-coil, actuated by two Bunsen-cells, the conducting-wires 
being connected alternately with the electrodes and with the 
tinfoil coatings. In the latter case, of course, the current of 
the induction-coil cannot penetrate into the glass; but the 
coatings at the moment of opening or closing the induction- 
current become charged, the one with positive, the other with 
negative electricity, and these charges disappear again equally 
rapidly. This continual charging and discharging of the 
coatings on the outside of the tube produce (as soon as the 
atmosphere of the tube is sufficiently exhausted) induced 
currents inside the tube, which show their presence by lumi- 
nosity. This experiment, which, like all the rest, was made 
in a dark room, gave the following result :— 
Pressure in tube. 
1m. 
531. Hach interruption of the induced current produced 
a spark between a and 6, but no discharge was ob- 
served in the tube between c¢ and d. 
395. The same result. 
316. The same, except that the sparks between a and b 
are now more brilliant. 
166. 
104. | There is still no discharge between ¢ and d, but the 
58. sparks between a and b become very brilliant. 
36. 
1. The luminosity between c and d becomes visible, and 
the spark between a and b becomes still more brilliant. 
0:12. The luminosity between ¢ and d is greater than 
before. The whole tube becomes luminous when the 
current passes from a to b. 
0:017. Nearly the same result. 
0-004. The current still passes from a to b, but with 
 feebler light ; intense luminosity between c¢ and d. 
0:00036. The spark between a and b is seen only occa- 
sionally, the tube remaining dark for long intervals of 
time. The induced light between cand d, on the other 
hand, is to be seen all the time, and is very intense. 
