168 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
Selecting the observations with the current positive in each case and placing these 
in juxtaposition thus — 
C 
V 
It 
Case I. 
0-0007261 
1730 
2,383,000 
„ II. 
0-0003674 
1756 
4,780,000 
we see that when C is varied in the ratio of 2:1, V remains sensibly constant, It 
varying as 1:2; that is to say, though the current is halved the difference of 
potential between the ends of the tube remains constant — a condition which could 
only be brought about when metallic resistance is substituted for the tube, by 
doubling this resistance. 
This points to the important conclusion that other things being Jcept constant and 
the current alone varied, we should find the value of V strictly constant for all values 
of C ; but it may readily be imagined that in experiments with 1 vacuum tubes ’ it is 
not easy to ensure perfect constancy of the accompanying circumstances. 
To test this conclusion we extended the range of our observations by varying the 
value of C as much as from 1 to 135. We give below the original measurements 
themselves, not the mean results, in order that the discrepancies in the readings 
obtained for V when C was kept as constant as our powers of control permitted, may 
be compared with the variations, such as they are, in the values of V when the circuit 
was purposely varied so as to produce currents of different strengths. These observa- 
tions show clearly that discharge through rarefied gases cannot be at all analogous to 
conduction through metals ; for a wire having a given difference of potential between 
its ends can permit one — and only one — current to pass ; whereas, we see from the 
following measurements that with a given difference of potential between the 
terminals of a given vacuum, tube, currents of strengths varying from 1 to 135 can 
flow. We are therefore led to the conclusion that the discharge in a vacuum tube 
does not differ essentially from that in air and other gases at ordinary atmospheric 
pressures — that it is, in fact, a disruptive discharge.* 
Fig. 42. 
Tale 31 
See Appendix, note B. 
