520 
Mil 0. Ml RICHARDSON ON THE ELECTRICAL CONDUCTIVITY 
was replaced by tlie tulje shown In fig. 6. In commencing an experiment the 
apparatus was exhausted to ’001 mlllim., so that the gas which was afterwards in the 
tube was all given off from the hot filament and the walls of the tube. The 
observations rvere generally taken so as to keep the tube as cool as possible, but by 
letting It get hot enough pressures up to a millimetre could be legistered on the 
McLeod gauge, even with constant ]:)innping. The first experiments were made to 
determine the way in which the leak varied with the applied electromotive force, 
other conditions being, so far as possible, kept constant. 
galvano- 
§ 2. Relation hetireen the Current and the Applied E.M.F. 
In all cases there was no current which woidd shov^ a deflection in the 
meters used when the filament was charged })ositively. Tlie positive leak from hot 
wires in a vacnnm, though large when measured fjy an electrometer, is always 
negligible compared with tlie currents measnied In these experiments. Some of the 
snl)joined current E.M.F. curves were olffained by using the apparatus in fig. 6, others 
by using that In fig. 7. As we should expect the curves to vary considerably with 
the shape and position of the electrodes, the apparatus from whicli the curves were 
detained will be definitely specified in each case. As an abbreviation for the 
“apparatus shown in fig. G” we shall write “apparatus 6,” and so on. 
The relation between the current and the electromotive force depends largely on 
the pressui'e of the gas in the apparatus. It may also depend on tlie value of the 
maximum current which can be ol^tained, i.e., on the temperature of the wire. At 
very low pressures (Ijelow, say, '02 inilhni.) the cnrrent rises very rapidly with the 
E.M.F. till it reaches a certain value, after which it becomes practically independent 
of the E.M.F. Tliis “ saturation cnrrent” generally increased slightly with the electro- 
mcttlve force, tlie Increase being attrilintable to the extra ions produced by collisions 
with tlie gas molecules. The following curve, given by apparatus 7, shows the 
phenomenon of saturation very clearly. The 
fiow^ of the heating cnrrent was accompanied 
by a P.D. of 3’8 volts between the two ends of 
the filament, so that it is important to state 
wliich end of the filament the voltages refer 
to. There was found to lie no cnrrent wdieii 
the negative end of the filament wms earthed, 
the Avhole of the filament being then positive 
to the snrronnding earthed electrode. The 
values are the means of a considerable number 
of observations. This curve (fig. 8) is for a 
pressure ol '003 millim. ; the voltage given is that of the negative end of the filament. 
From the preceding curve it will be seen that the cnrrent wms practically saturated 
by a potential of about 15 volts. With higher pressures of gas in the apparatus, the 
10 ZD 30 40 50 60 70 80 90 100 llO 120 
Volts on negative end of filament 
Fig. 8. 
