at Low Pressures. 
257 
At pressures below 1 millimetre X is nearly independent of 
the current for the small range of current which can be employed. 
For the purpose of comparing the values of X at different 
pressures I have used the value of X corresponding to the 
smallest possible steady current at each pressure. When the 
current is nearly as small as possible X is nearly independent 
of the current except at the higher pressures, where it always 
diminishes slowly as the current is increased. 
Table VI. shows the way in which X varies when the pressure 
( p ) is changed. The results show that ^j- is nearly a constant 
for pressures from 0‘20 to 2'82 mms. 
X = 34'9 Vp. 
Table VI. 
Pressure 
Electric Intensity 
Z 
sIV 
0-200 
16-05 
35-9 
0-275 
19-7 
37-6 
0-310 
20-5 
36-9 
0-383 
22-7 
36-7 
0-550 
26-3 
35-5 
0-583 
26-3 
34-5 
0-792 
28-4 
32-0 
1-08 
33-4 
32-3 
1-13 
31-8 
33-6 
1-52 
42-9 
34-9 
2-05 
49-2 
34-5 
2-08 
50-8 
35-3 
2-82 
57-0 
34-0 
Mean 34-9 
There is another way in which the apparatus described can be 
used to measure the Hall effect. After applying the magnetic 
field in one direction the electrodes EE' are rotated until they 
are both at the same potential. The field is then reversed and 
the electrodes again rotated till their potentials are equal. The 
