Duane — Emission of Electricity. 5 



were made to test this point. For instance, in one series the 

 positive current measured with a liquid air vacuum .was 3'04. 

 On allowing air to enter the tube to a pressure (measured 

 roughly by a small mercury manometer) of about l-5 mm of 

 mercury, the current six minutes later rose to 3*21. Six min- 

 utes later still the liquid air vacuum having been reproduced, 

 the current fell to 2'34. Correcting for the decay of the cur- 

 rent by taking the mean (2*69) of the first and last values, we 

 see that the ionization in the air at l-5 mm of pressure increases 

 the current from 2*69 to 3-21, i. e., less than 20 per cent. A 

 number of such experiments showed that the currents due to 

 ionization in air between A and B at pressure of 1 to 2 mm 

 were somewhat less than the currents to be measured, and it 

 follows that the infinitesimal quantity of air left after exhaus- 

 tion by the liquid air process can have no appreciable effect 

 on the currents. This is to some extent due to the fact that 

 the surfaces of the tube and electrode were very close together. 

 In fact, the apparatus was designed to minimize the effect of 

 ionization in the air as much as possible. 



9. The fourth column in Tables I and II contains the values 

 of i the algebraic sum of the positive and negative currents. 

 This % algebraic sum represents roughly the discharge per 

 second of negative electricity from the electrode in zero electric 

 field less the negative electricity received by the electrode per 

 second from the surface of the tube. Let e be the difference 

 between these two currents, and dzi be the current produced by 

 the applied electromotive force : i represents, then, the effect 

 of the impressed electromotive force on the slow-moving 

 electrons projected from the metallic surfaces. When the 

 tube is charged positively, the positive current is 



\ = e + i, (1) 



and when charged negatively, the negative current is 



*, = e - i, (2) 



and. evidently 



2 'i + \ 

 e = — 



2 



10. The above is the usual method of analyzing the currents 

 in similar cases, but the theory contains two assumptions, first 

 that the small electromotive force (2'2 volts) does not percepti- 

 bly change the value of e, and second that the current i has 

 the same absolute magnitude whichever way the electromotive 

 force acts. It is probable that neither of these assumptions is 

 absolutely correct in the present case, but that the equations 

 approximately agree with the facts may be seen from the 

 following experiments. 



