IMPARTED TO A VACUUM BY HOT CONDUCTORS. 
529 
filament melted. The arrangement of the apparatus was altered somewhat, the leak 
being measured by a Weston ammeter instead of a galvanometer as before; in other 
respects the arrangement was unchanged. The pressure was kept very low and a 
potential of about — 60 volts was maintained on the filament, the surrounding 
cylinder being earthed. 
W^ith this apparatus it was found possible to maintain an actual current of 
•4 ampere (corresponding to 2 ampeees per square centimetre of filament surface) at 
a pressure of less than millim. The current could not be made to surpass this 
value since the filament melted on raising it to a slightly higher temperature. The 
fact that such large currents can be produced at such low pressures has an important 
bearing on the theory ol the mechanism by which the corpuscles are produced, which 
will be considered later. 
§ 5. The Relation hetiveen the Saturation Current and the Temperature. 
Fiom the experiments on the variation of the saturation current with the 
resistance we have been able to give numbers which indicate, roughly at any rate, 
the way in which the former depends on the temperature. It was thought that a 
moie leliable estimate might be obtained if the temperature of the filament were 
determined by means of a thermal junction of 2 Dlatiniim and iridio-platinum. With 
this object the following exjjeriments were made :— 
The tube employed was that shown in fig. 7, and already described, The filament 
in this tube was in the form of a simj^le U and had the following linear dimensions : 
length = 1-2 centim., diameter = -0376 centim., and total area of surface = -142 sq. 
centim. For these experiments the apparatus shown in fig. 2 had to be altered, the 
portions below AFK. being entirely reconstituted. The apparatus used for measuring 
the saturation current was unchanged, the only alterations being made in the portion 
used to measure the temperature. The thermocouple circuit was completed by 
taking the lead Pj (fig. 7) to the cold junction, which was placed in a test-tube 
immersed in water at 12° C. ; the other wire from the cold junction passed through a 
resistance box to a D’Arsonval galvanometer, and thence through P (fig. 7) to the 
hot junction. The adjustable resistance (fig. 2) still served to regulate the 
current which was used to heat the filament. 
In order to standardise the thermocouple the melting-point of potassinm sulphate 
was again taken as the fixed point. A junction of the same wire as that used during 
the experiments was fixed on to a stout platinum wire, which was clamped horizon¬ 
tally in the hottest jDart of a Bunsen burner. I'he Bunsen was arranged to burn 
vigorously with a bright green inner cone and was carefully protected from draughts. 
Very small portions of the salt were then placed on the stout wire on the side of the 
flame opposite to the thermocouple, and matters were so arranged that when the salt 
just melted it was exactly the same distance from the edge of the flame on the one 
side as the thermocouple on the other. I he reading of the galvanometer was then 
YOL. CCI.—A. 3 Y 
