IMPARTED TO A VACUUM BY HOT CONDUCTORS. 
519 
ciUTent required to heat it, respectively, on the other. The variation of the 
saturation current with the resistance gives an approximation to its variation with 
the temperature of the filament. It was thought that a better estimate might be 
obtained if the temjierature of the incandescent 
filament were determined by means of a thermo¬ 
couple. With this object a second form of tube was ^ 
set up, which we shall proceed to describe. 
In this case the filament F, together with the 
platinum wires which support it (see fig. 7), was cut 
out ot the original lamp and fastened to two stout 
copper wires G and G^. This was done by placing 
the platinum and copper wires alongside, wrapping 
them round closely with fine copper wire and then 
soldering the whole. The filament was thus sup¬ 
ported on two long copper legs ; the rigidity of the 
structure was ensured by melting on two cross- 
jiieces of blue glass in the jiositions shown in the 
figure. The ends of the copper terminals rested in 
the small tubes, Tg and T^., wliich contained mercury, 
and which were fused in to the end of the large 
tube B. The curi'ent which heated the filament 
entered Ijy platinum wires, which were melted into 
the tubes Tg and T^. The electrode, to which the 
current was measured, was a long narrow aluminium 
cylinder E, which practically surrounded the hot 
filament. Tlie cylinder was supported by a stout 
wire let in through the side-tube A. The tube D 
was connected with the pump and McLeod gauge. 
The thermocouple was of platinum and iridio- 
platinum, the wires being the finest obtainable. 
The pure platinum wire was '0025 centim. in diameter, whilst the 10 per cent, 
iridium alloy had a diameter of ‘0035 centim. The wires were tied together on 
to the filament by means of a slip-knot, so as to make good contact but not to 
increase the diameter of the filament materially. They were then suspended from 
platinum wires let in to the tubes T^ and To, which were inserted in the tube 
LG in exactly the same way as Tg and T^, were fixed in to B. The wires P and P^, 
which were prevented from touching by the cardboard partition H, connected the 
mercury cups d\ and Tg with the rest of the thermocouple circuit. The tubes B and 
C were connected by a sealing-wax joint S just as in the former apparatus. 
In the first experiments with cai'bon the apparatus shown in fig. 2 was used, just 
as for platinum, except that the tube with the incandescent platinum wire (fig. 1) 
Fig. 7. 
