482 TATE AND FOOTE : ANODE FALL OF POTENTIAL 



sufficient to cut the effective speed, of even as small a pump as 

 the one described above, down to half, while a pump capable of 

 infinite speed working through such a connection could only do 

 as well as could the pump described above without such connect- 

 ing tube. 



When the time of initially reaching a sufficiently low pressure 

 for the high vacuum pump to be effective becomes of importance 

 the fore pumps can be made faster; however, this element is not 

 often of so much consequence, for even a small water aspirator, 

 working properly, in conjunction with the intermediate pump, 

 will, in about five minutes, pump a volume of one liter down to 

 a pressure sufficiently low for the high vacuum pump to begin 

 effective operation and will pump larger volumes in proportion- 

 ately longer times. 



PHYSICS. — Probe-wire measurements of anode fall of potential. 

 John T. Tate and Paul D. Foote, Bureau of Standards. 



In connection with some work which the writers have in 

 progress it was necessary to observe the amount of energy 

 dissipated at the anode in a Wehnelt discharge tube. For 

 this purpose a probe wire, or sound, was inserted in the anode 

 glow and measurements were made upon the current flowing and 

 upon the potential drop between the anode and sound. 



In discharge tubes of this type it is easily possible to secure 

 currents of several amperes with an anode fall of twenty volts 

 or less. If the probe-wire circuit may be considered as obeying 

 the ordinary laws of metallic circuits, it might at first appear 

 that the resistance between the probe and the anode is small 

 enough to permit accurate potential measurements by use of a 

 high-resistance voltmeter instead of an electrometer. It was 

 found, however, that the voltmeter readings were quite different 

 from the electrometer readings. 



Two explanations of this fact may be offered. That the 

 probe wire on open circuit actually takes up the potential of 

 the space surrounding it has been fairly well established. 1 On 

 closed circuit the probe either takes up the potential of the 



Skinner. Phys. Rev., 9: 97. 1917. 



