252 Mr Whiddington, On a mechanical vacuum tube resin 
mounted so as to project well within the bulb. The glass sheath | 
was closely fitting and was controlled magnetically. It is im-— 
portant to select the glass sheath so that it is entirely free from 
air lines as otherwise the discharge passes along the lines and the | 
tube runs unsteadily. The tube was evacuated to a convenient 
point and driven by a Mercedes influence machine which proved 
a fairly steady source of current, the voltage across the tube being | 
measured by a Braun electrostatic voltmeter. In the graph the — 
abscissae represent lengths of sheath outdrawn while the cor- | 
responding ordinates are the corresponding voltages across the | 
tube. The pressure of gas within the tube was maintained con- | 
stant throughout the experiment. Accurate readings could not | 
Working Voltage 
Oienie” 5 10 ney: 20 25 | 
Length of Sheath (in mms.) 
(Diameter of Cathode=15 mms.) 
be continued above 10,000 volts with the voltmeter but an | 
equivalent spark gap indicated that the tube at its hardest was | 
working at about 40,000 volts, beyond this point the tube became — 
very unsteady in action. 
It is noteworthy that beyond the point P on the graph the 
curve rises far more steeply. Observations carried out to above 
10,000 volts with a spark gap indicate that the curve continues 
practically straight up to about 30,000 volts. 
It seems very difficult, if indeed it is at present possible, to 
give an adequate explanation of the action of the sheath. This 
is a difficulty common to very many problems in connection with 
discharge tubes. What I propose to do then is to give a brief — 
