528 
MR. 0. W. RICHARDSON ON THE ELECTRICAL CONDUCTIVITY 
It will be noticed that there is a great similarity between fig. 14 and the curves 
connecting the saturation current with the resistance and temperature respectively. 
I’his is merely due to the rate of variation of the saturation current being so rapid 
that the differences in the alteration of resistance, temperature, and heating current 
becomes insignificant in comparison. 
Since in the last experiments the big currents were always accompanied by a high 
pressure of gas in the a 2 )paratus, it inight be thought that part of the increased 
current was due to the gas present. To investigate this point a series of experiments 
was made with the apparatus shown in fig. 7, so that the gas pressure could be kept 
down to a very low value. By heating the filament for a short time only and taking 
the observations very (quickly, it was found that the temperature of the bulb and 
electrode could be prevented from rising perceptibly. Under these conditions it was 
found that the amount of gas given off was greatly diminished, the highest pressure 
recorded during the observations being ‘006 millim. A reading of the McLeod gauge 
was taken between each reading of the galvanometer deflection for the saturation 
current. The potential on the filament was — 80 volts, this being more than enough 
to saturate the current [cf. fig. 8). The numbers which were obtained are given in 
the accompanying table : 
Heating cuiTeut, 
amperes. 
Saturation current, 
1 = ampere x 10“**. 
Pressure, millims. 
of Hg. 
1-49 
2-1 
•002 
1-59 
10-5 
•002 
1-70 
35 
•0025 
1-84 
143 
•0025 
2-0 
540 
•003 
2-26 
2•24X103 
•003 
2-43 
10-1 xl03 
•005 
2-68 
42 X103 
•005 
2-92 
122 X103 
•006 
3-45 
760 X103 
•006 
3-65 
1640 X103 
•005 
The greatest observed value of the saturation current is not given in the above 
table, since the corresponding reading of the ammeter was not taken. This enabled 
the reading to be taken much more cpiickly, so that the pressure only changed from 
■0022 to "0025 millim. The corresponding saturation current was ’04 ampere; in 
other words, a square centimetre of surface would have given a current of ‘28 ampere 
across a vacuum at 4 ^oth millim. pressure. 
In the preceding series of experiments the highest possible value of the temperature 
had not been rea.ched, so that further experiments were instituted to determine the 
maximum cui-rent which could be obtained from a square centimetre of a carbon 
fixament when the temperature was pushed to the highest limit, fie., just before the 
