1878.] 



Discharge in Exhausted Tubes. 



379 



electrometer, the differences of potential between the electrodes of the 

 tube, and between the ends of any convenient known resistance in the 

 same circuit. 



Fig. 4. 



We were induced to extend the investigation by means of the elec- 

 trometer, to the determination of the rate of fall of the potential 

 throughout the length of a vacuum tube. For this purpose we had 

 several tubes constructed with aluminium rings fixed to platinum 

 wires inserted in the glass at nearly equal intervals. It was found 

 that the fall of potential for equal spaces had a maximum value in the 

 neighbourhood of the negative electrode ; in the neighbourhood of the 

 positive, though considerably less than near the negative, it was greater 

 than in the middle of the tube, where for equal intervals the fall of 

 potential was sensibly constant. 



The most important result of these measurements was the establish- 

 ment of the constancy of the difference of potential between the elec- 

 trodes of a given tube and gas, at a given pressure, for all variations 

 in the value of the current, even when these were as great as 1 to 

 150. This conclusion was arrived at subsequently to the observation 

 of the majority of the phenomena recorded in the paper. Accordingly, 

 it must be understood that what has been termed " resistance of a 

 tube " indicates that the difference of potential between its terminals 

 was the same as that which would have existed between the ends of a 

 metallic wire of the resistance given if substituted for the tube in the 

 particular circuit. 



We have, moreover, by experimenting with such tubes as 154 and 155, 

 fig. 5, wherein there is a constricture of the same bore, but of different 

 lengths, established the fact that the constricture greatly increases the 



vol. xxvii. 2 c 



