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 —r is nearly a constant 

 for pressures from 0'20 to 2*82 mms. 



Table VI. 



Pressure 



Electric Intensity 



X 



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 



353 



2-82 



57-0 



340 



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 



