256 
DR. HAROLD A. WILSON ON THE 
wire itself. It seems therefore that the results obtained are consistent with the view 
tliat N varies inversely as the absolute temperature. 
The dotted curve in %. 4 was calculated by means of the expression 
log nijUa = 
V 
E log hja 
) — (NEpa/V)log i)/a 
1 t 
— e 
— (NEp6,rV) log bja 
\ 
eoo 
400 600 
Pff£S%uRE 7nms 
Fig. 5. 
800 
takino- N = 8-04 and E = 16-55. These values were obtained by making the 
iiiasimum on the calculated curve coincide with tlie observed maximum. Below 
4 millims. pressure the oloserved and calculated curves agree fairly well, but at the 
higher pressures the calculated current is 
too large. At high pressures most of the 
ionization by collisions takes place near 
the wire where the gas is very hot, so 
that the value of N ought really to be 
taken smaller at the higher pressures, 
consecpiently the calculated currents are 
too large. 
Fig. 5 shows the variation of the nega¬ 
tive leak with the pressure using 40 volts 
only, so that practically no ions are produced by collisions. The current falls off as 
the pressure increases. This is probaljly due to the presence of the gas hindering the 
escape of the negative corpuscles from the surface of the wire. 
Some measurements of the negative leak were also made in nitrogen and in water- 
vapour. The nitrogen was prepared from KNO^ and NH 4 CI and passed over HoSO^ 
before admitting it into the apparatus. The last traces of oxygen were removed from 
it by sodium electrolysed into the apparatus througii hot glass. The details of this 
process will be described in Section (4). The leak in nitrogen at pressures up to several 
millims. was nearly e(.|ual to that in a good vacuum from similarly treated wires, 
provided tlie P.D. used was not large enough to produce ionization l.^y collisions. The 
effects with large P.D.’s were similar to those obtained in air. 
The leak in water-vapour at temperatures Ijelow 1500° C. with a pressure of about 
15 millims. was also very nearly equal to that in a good vacuum at the same 
temperature. The tube containing the wire was connected to a three-way tap, which 
was also connected to a tube leading to the pump and to a small bulb containing 
water. The tube and the bulb were exhausted separately and then connected 
together, several times, so that the tube containing the wire was tilled with water- 
vapour. The current obtained was 25 divisions at 1446° C. The corresponding 
current in a vacuum was 27 divisions, using a wire wliich had been treated in a 
similar way. 
The positive leak in air and other gases is very small at low pressures. When a 
platinum wire which has not been specially cleaned is first heated in a vacuum or in 
