Mr Almy, On a Diminution of the Potential Difference , etc. 187 
electric machine to furnish the discharge, so that the current could 
be measured. A D’ Arson val galvanometer served as current-meter. 
Potential measurements were made by the absolute (Kelvin) 
electrometer. For the measurement of potential differences 
several times as large as the range of the instrument, a very 
high resistance, a long tube of xylol with several platinum 
electrodes at points along the tube, was connected in shunt to the 
discharge tube, and the potential drop over a part of this was 
read off with the voltmeter. The relation of a part to the whole 
was readily determined by observing the potential differences 
between the electrodes of the different sections of the resistance, 
with a constant total and summing the parts. 
The following series of observations, taken each with constant 
current, show the character of the variation of the p.-d. between 
the electrodes of a tube, with variation of cathode-area : 
Table V. 
I. 
II. 
III. 
IY. 
V. 
Gas 
i pressure 
12 mm. 
12 
0-12 
0-08 
0-06 mm. 
Current . 
1-0 
50 
60 
42 
50 
,0T2 
3000 
3600 
36000 
eS 
03 
0-28 
^ « | 
1500 
2300 
11200 
29400 
Sj 
o3 
0-78 
•1 § 
1300 
1800 
7000 
25200 
32300 
03 J 
'"C ' 
1-77 
§ £ \ 
1000 
1300 
5600 
23800 
26400 
o 
2-54 
800 
1150 
6600 
22000 
30000 
<3 
3-80 
Pm "S 
700 
1000 
8700 
28600 
37000 
o 
V 5-30 
\ 
600 
800 
13000 
32200 
41000 
In the discharge of the Wimshurst, at low pressures, there 
appears a noteworthy phenomenon. The character of the discharge 
is of two distinctly different types. With cathode area small the 
discharge takes place in streams, the discharge from the cathode 
seeming to come off mainly from certain points or small spots, 
at which a luminescence occurs, the discharge having a con- 
siderable similarity to brush discharge from a point at atmo- 
spheric pressure. Then, as the cathode area reaches a somewhat 
larger value the discharge passes to the steady glow or dark ” 
discharge common in the Crookes’ tube at low pressure. 
So long as the first type of discharge takes place the potential 
difference between the electrodes decreases as the cathode area 
increases, but from the point at which the character of the dis- 
