ELECTRIC DISCHARGE WITH THE CHLORIDE OF SILVER BATTERY. 479 
In both of these series of experiments it will be seen that the pressure of minimum 
resistance for an air partial vacuum in the vessel of spheroidal shape and of the 
dimensions mentioned is about 3 m.m., 3947 M, corresponding to the pressure of air 
at an altitude of 27 - 42 miles, at which height the aurora would from these data have 
a maximum brilliancy and would be visible at a distance of 499 miles. This result is 
very different from that before cited, which was obtained with a hydrogen tube from 
which the pressure of maximum brilliancy was deduced to be for air 0\379 m.m., 
499 M.,"“ at a height of 37’67 miles and visible at a distance of 585 miles. 
As no direct experiments had been made for an air residual vacuum in smaller 
tubes, the following investigations were made with a tube, No. 342, 1’625 inch 
(41 centims.) in diameter and 22'5 inches (57 centims.) between the terminals, one 
a ring positive, the other a point negative (see fig. 2, on the same scale as fig. 1). 
Battery 11,000 cells, deflection when short-circuited 70°, showing a total internal 
resistance of 151,000 ohms. At a pressure of 19 m.m. there was a glow on each 
terminal, but the current was not sufficient to deflect the galvanometer used until 
it had been lowered to 14 m.m. 
Pressure. 
Deflection. 
Total 
resistance. 
Tube- 
resistance. 
Tube- 
potential V. 
m.m. 
M 
Cube root of M. 
ohms. 
dims. 
cells. 
14-0 
18,421 
26-41 
46 
400,300 
249,200 
6848 
12-0 
15,789 
25-09 
55 
291,200 
140,100 
5291 
11-0 
14,474 
24-37 
55 
10-0 
13,158 
23-59 
55 
9-0 
11,842 
22-79 
56 
280,400 
129,300 
5072 
8-0 
10,526 
21-58 
57 
263,600 
112,500 
4693 
7-0 
9211 
20-96 
60 
239,500 
88,400 
4061 
5-0 
6579 
18-74 
61 
230,200 
79,100 
3780 
3-0 
3947 
15-80 
62 
220,900 
69,800 
3477 
2-0 
2632 
13-81 
62 
0-69 
908 
9-68 
63 
211,600 
60,500 
3146 
0'23 
303 
6-72 
62 
220,900 
69,800 
3477 
0T5 
197 
5-82 
60 
239,500 
88,400 
4061 
0-05 
66 
4-04 
57 
263,600 
112,500 
4693 
0-03 
39 
3-39 
54 
302,100 
151,000 
5499 
The pressure of least resistance is therefore 0'69 m.m., 908 M., in the tube in 
question, and it does not differ materially from 0'642 m.m., the pressure of least 
resistance in hydrogen. 
A pressure of 0'69 m.m. corresponds to an atmospheric height of 34'7l miles, and 
an aurora at this height would be visible at a distance of 561 '4 miles. 
* Proc. Roy. Soc., Ho. 203, 1880. 
3 Q 2 
