422 Dr Frank Horton, The ionisation produced by certain 
figure being obtained on the supposition that the part of the 
filament which had been covered with phosphate was still covered 
when these observations were made. Since the current per 
sq. cm. calculated on this supposition is rather less than that 
given by platinum coated with sodium phosphate at a much 
lower temperature, it seems probable that in places the salt had 
volatilised away from the filament, so that the area covered when 
the observations were taken was really much less than that 
assumed in the calculation. The fact that the maximum emission 
is obtained at a lower pressure in the case of the Nernst filament 
is in agreement with the general characteristic of the positive 
emission from glowing solids, namely, that the pressure of 
maximum emission is lower the higher the temperature of the 
anode. 
After these experiments with sodium phosphate, the filament 
was taken down and repeatedly washed in distilled water to 
remove any remaining sodium salt. It was then replaced in the 
apparatus and the positive emission at 1422° C. was again tested. 
The current-pressure curve obtained is also shown in fig. 2. 
This curve is similar to that given by a new filament after 
heating to a steady state, but the values of the currents shown im 
the curve are rather larger than those generally obtained. The 
difference in the form of this curve and that obtained with sodium 
phosphate on the filament is very marked. A similar difference 
is shown by the curves for platinum covered with sodium phosphate ~ 
and for platinum alone. Evidently the increase of the emission to 
a maximum value at a few mm. pressure is due to the presence 
of the sodium salt on the anode; and the large emission obtained 
from sodium phosphate heated upon platinum is due to the salt 
itself and not to the platinum. 
Dr W. Wilson, in his experiments*, could detect no positive 
emission from a Nernst filament covered with aluminium phos- 
phate, but he obtained an emission while the heater connected 
with the filament was kept glowing, and he concluded that 
the latter effect was due to the fine platinum wire of the heater. 
In an earlier paper the author has shown that the activity of 
ordinary aluminium phosphate is due to sodium impurity which 
would immediately sublime away at the high temperature of the 
fully glowing filament, but would probably give a steady emission 
at the much lower temperature of the heater; so that the current 
obtained in Wilson’s experiments while the heater was in operation 
may have been due to the material of which it is made, or to some 
of the phosphate which had fallen upon it. 
The form of the current-pressure curve given by salts contain- 
ing sodium is very interesting. In my paper on the discharge of 
* W. Wilson, Phil. Mag. 6, xx1. p. 634, 1911. 
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