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td substances when heated on a Nernst filament. 415 
been shown that sodium phosphate heated upon platinum gives 
a large positive ionisation which is more permanent than that 
given by aluminium phosphate™*. 
The discharge tube used in these experiments is similar to 
that described in an earlier paper*, the only difference being 
that the two parallel platinum plates which form the anode are 
rather further apart in the present apparatus. These plates are 
situated vertically, 1:5 cms. apart, at the centre of the small bulb 
which forms the discharge tube. The Nernst filament when in 
position is parallel to the “plates and mid-way between them. It 
can be heated by an alternating electric current led in through 
stout platinum wires, and the filament and its leads can easily be 
removed from, or replaced in, the bulb. The alternating current 
‘was obtained from the secondary of a transformer, the primary 
‘coil of which was connected to the alternating town supply (100 
ili), and the current through the filament could be varied by 
changing the resistance in both primary and secondary circuits. 
‘One of the fine iron wire resistances supplied with Nernst lamps 
was always kept in series with the filament. As this resistance 
en a large positive temperature coefficient, it tends to keep the 
temperature of the filament more steady than would otherwise be 
the case. 
_ In order to start the filament glowing it was ‘clean From the 
discharge tube and heated by holding it above, and near to, 
a glowing “ heater” of the kind supplied with an ordinary Nernst 
lJamp. It was then placed in the apparatus which could be 
rapidly exhausted, when required, by means of a water-pump, 
mercury-pump, and charcoal tube cooled in liquid air. In this 
way the gas pressure in the apparatus could be reduced to 
‘0001 mm. within twenty minutes from the time that the glowing 
filament was placed in position. 
_ The temperature of the filament was determined by means of. 
a Féry optical pyrometer, which was very kindly lent to the 
evervlist Laboratory by Professor T. Mather, of the City and 
Guilds College, London. This instrument was standardised by 
using a platinum tube of about the same diameter as a Nernst 
filament, with a thermo- couple of fine wires of platinum and 
platinum- rhodium welded to it. In order to have the surface 
of the tube exactly similar to that of the filament, a filament 
was finely powdered and mixed with water, and the platinum 
‘tube was then covered with a thin layer ‘of Nernst filament 
material by evaporating this mixture upon it. This platinum 
tube was fitted up in the place of the filament in the discharge 
bulb, and observations of the thermo-electromotive force and 
* F. Horton, Proc. Roy. Soc. A. Lxxxviil. p. 117, 1913. 
