substances when heated on a Nernst filament. 421 
obtained with decreasing pressures are plotted as a curve in 
fig. 2. In the following table some results at various pressures, 
1:94 x 10-® ampere 
Current: 1 
0 10 20 30 : 40 50 
Pressure mm. 
Fig. 2. 
both increasing and decreasing, are given. The close agreement 
between the values in the two series shows that the emission had 
really been reduced to a steady state. 
| 
Pressure (mm.) 0024 | 13:25 15-0 | 45:0 = 56°5 
Thermionic current : | 
Increasing pressures 92 160 163 en | ra! 
Decreasing pressures 95 162 160 120 114 
The curve in fig. 2 is similar to that which represents the 
connection between positive ionisation and pressure for sodium 
phosphate heated upon a platinum strip. In both cases the 
emission at first increases with decreasing pressure to a maximum 
value at a pressure of a few mm., after which it decreases rapidly 
as the pressure is still further reduced. In some earlier experi- 
ments* the maximum value of the positive emission from sodium 
phosphate heated upon platinum at 1190° C., under an applied 
potential difference of 200 volts, was 2°25 x 10° ampere per 
sq. cm., and occurred at about 10 mm. pressure. From the curve 
in fig. 2 the maximum current is obtained at about 3 mm. 
pressure, and its value is 1:97 x 10-* ampere per sq. cm., the latter 
* loc. cit. 
