IMPARTED TO A VACUUM BY HOT CONDUCTORS. 
507 
B.—Experimental Investigation. 
I. Experiments with Platinum. 
§ 1. Description of the Apparatus. 
The ultimate object of the experiments was to determine the way in which the 
saturation current from a hot platinum wire to a surrounding electrode, both placed 
in a vacuum, varied with the temperature of the wire. For this purpose the type of 
bulb shown in fig. 1 was found to be most convenient. 
The wire to be heated was in the form of a spiral, with its axis passing centrally 
along the length of the tube ; the current through the wire was supplied by means 
of the two thick leads AA^ and The electrode to 
which the current was measured was an aluminium 
cylinder which surrounded the hot part of the wire, 
dhe cylinder was supported by a stout aluinimuin wire 
E, sealed through the side tube D by means of platinum. 
Ihe end E was connected to the electrometer or galvano¬ 
meter which served to measure the current. The side 
tube F connected the bulb with the pump and McLeod 
gauge. 
In the earlier experiments, trouble was experienced 
owing to loose contacts appearing at A^ and when the 
platinum wire had been heated. In the final form of the 
tube this was obviated by making the leads AA^ and BB^ 
of platinum wire 1 niillim. thick, to which the ends of 
the platinum spiral A^B^ were welded electrically. Tliis 
made the platinum quite continuous through the tube. 
Ihe support E of the electrode CC| was insulated out¬ 
side the tube at D by means of sealing wax. Inside the 
tube there was only glass insulation, which, however, is 
very good at low pressures. 1- 
The temperature of the platinum wire was olitained by measuring its resistance. 
The arrangement of apparatus which was used to do this and to measure the current 
from the surface of the wire is indicated in fig. 2. The whole of the apparatus below 
AI Kj IS the part which was used to determine the resistance and was insulated on 
paraffin blocks. It could be charged to any desired potential up to 400 volts by 
means of the battery B^ through the key and water resistance A. The potential 
was measured by the vMt-meter W. In this way any desired potential could be 
maintained on the hot wire F. The cylindrical electrode C was put to earth through 
the galvanometer G^, which thus served to measure the current. In some of the 
3 T 2 
