232 Mr. G, Owen on the Discharge of 
turns. The secondary employed to supply the current through 
the filament had 151 turns, while that used to heat the 
spiral contained 50 turns. As the secondaries were fairly 
well insulated, the filament could be raised to any desired 
potential by connecting it to one pole of a battery of small 
storage-cells, the other pole being earthed. 
By means of this preliminary apparatus it was found that 
there was a large negative leak from the glowing filament at 
atmospheric pressure, together with a much smaller positive 
leak. It was further found that the filament could be kept 
lowing in a vacuum without much vapour being evolved, pro- 
vided that a large resistance (about 100 ohms) was inserted 
in series with the filament so as to prevent the temperature 
reaching too high a value. If the temperature of the fila- 
ment exceeded a certain value, large quantities of some 
metallic vapours were evolved (at low pressures) which 
condensed in a thick black deposit on the walls of the tube. 
In all the experiments to follow, the temperature of the 
filament was kept too low for this evolution of vapour to take. 
place to any considerable extent. 
It may be mentioned that the filament could not be made 
to start glowing in a vacuum, the heat radiated from the 
incandescent spiral not being sufficient to raise the tempera- 
ture of the filament to the critical value at which conduction 
commences. A pressure of 2 or 3 mms., however, was 
enough to supply the necessary conduction of heat. The 
platinum heater was therefore never heated except at fairly 
hign pressures, so that the purity of the air in the apparatus 
was not impaired to any extent by the gases given out from 
the glowing platinum. 
Having found that a good vacuum could be maintained 
when the filament is glowing, a new apparatus was made 
which would enable the value of e/m for the carriers of the 
electricity discharged from the filament to be determined. 
All the results given in the paper were obtained with this 
new apparatus, which is shown in figs. 1 and 2. Fig. 1 
gives a front view of the tube L, a side view of which is 
shown in fig. 2. The filament F, after having been cut 
together with its short copper leads from its porcelain base, 
was firmly fastened to iron wires rigidly connected together 
by a bar of lead glass (not shown in fig. 1) melted onto 
them. ‘The iron wires were in their turn attached to copper 
leads CC. AA are the wires of the thermal junction which 
served to measure the temperature of the filament, D is an 
earth-connected brass cylinder having two narrow slits 
parallel to the axis and reaching from one end of the cylinder 
