414 Dr Frank Horton, The ionisation produced by certain 
Lhe ronisation produced by certain substances when heated on| 
a Nernst filament. By Frank Horton, Se.D., St John’s College. 
[Read 9 March 191 4] 
The ionisation produced by heated solids has been investigated | 
by many observers in recent years, but at the present time none! 
of the theories which have been put forward to explain the) 
origin of the emission of either positive or negative electricity is: 
universally accepted. The substance upon which most experl-- 
ments have been made is platinum, but other metals, and many | 
compound substances, have also been used. As a rule, in testing | 
compound substances they have been supported upon a strip or | 
wire of platinum, and the view has been put forward that the 
observed effects are due to the metal support rather than to the | 
layer of substance under test. Thus certain oxides—those of the | 
alkaline earth metals—give a large emission of negative electricity | 
when heated upon platinum in an exhausted tube, and it has | 
been suggested that this effect is merely an increased electron 
emission from the platinum itself; the increase being caused by’) 
the lime lessening the energy required to enable an electron to | 
escape through the metal surface. The emission of positive 
electricity from various salts has also usually been studied when. 
the salt under test is heated upon a platinum strip. Dr W.. 
Wilson failed to detect any positive ionisation from aluminium 
phosphate heated on a Nernst filament, although the positive | 
emission from this salt when heated upon platinum has been | 
investigated by several experimenters. Dr Wilson has therefore 
concluded that “the leak observed when the salt is heated on | 
platinum is either mainly a leak from the platinum itself, or the | 
latter plays an important réle in its production*.” | | 
The author has recently been studying the thermal ionisation | 
produced by Nernst filaments and, having the apparatus at hand, | 
it was thought desirable to test these views as to the origin of the | 
ionisation from glowing solids by ascertaining (a) whether lime 
heated upon a Nernst filament gives a negative emission com- 
parable with that obtained when it is heated upon platinum, and | 
(6) whether the positive emission from a glowing Nernst filament | 
is increased. by placing sodium phosphate upon it. It has already | 
* W. Wilson, Phil. Mag. 6, xxt. p. 634, 1911. 
