in the Uniform Positive Column in Air. 



181 



The apparatus used is shown in fig. 1. A "vacuum-tube," 

 AB, 15 cms. long and 2 cms. in diameter, having an aluminium 



Fig. 1. 



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ro PUMf=> ETC. 



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disk electrode at each end was connected to a Topler pump^ 

 McLeod gauge, and bulb containing phosphorus pentoxide. 

 A side tube, CD, was joined on to the tube AB. This side 

 tube was about 15 cms. long and one sq. cm. in internal cross- 

 section. Two small platinum wires were sealed into it at E 

 and E^ and an aluminium electrode at D. A and B were 

 connected through a liquid resistance to a battery of small 

 secondary cells by means of which a discharge could be 

 passed between them. The electrode D was also connected 

 to a point, on the same battery, through a liquid resistance 

 and galvanometer. By adjusting the potential-difference be- 

 tween B and D a part of the current from B could be made 

 to pass down the side tube to D, and in this way a positive 

 column could be obtained in the side tube carrying any 

 desired current. 



The discharge from A to B prevented the discharge in the 

 side tube going out even when the current in it was very 

 small. In this way a positive column could be maintained 

 in the side tube with a current of less than 10~^ ampere. 

 With such small currents the light emitted by the positive 

 column was very faint, but it could be seen in a dark room 

 and it filled the tube with a faint uniform glow. The elec- 

 trodes E and E^ were connected to a quadrant electrometer 

 by means of which the P.D. between them when the dis- 

 charge w^as passing was measured. In this way the variation 

 of the electric intensity in the positive column in the side 

 tube with the current density could be obtained. The follow- 

 ing tables give the results obtained in air : — 



