IMPARTED TO A VACUUM BY HOT CONDUCTORS. 
533 
from metals which are low down in the volta series. In fact, assuming (1) that the 
difference in the discontinuity of potential at a platinum vacuum and sodium vacuum 
surface is equal to the contact difference of potential for sodium and platinum (taken 
roughly to be equal to two volts), (2) that the value of the discontinuity (Id volts) 
previously obtained for platinum is correct, (3) that the concentration of the 
corpuscles for sodium is of the same order of magnitude as for copper, and (4) the 
correctness of the present theory, a preliminary calculation showed that currents of 
the order of some 10“® ampere per square centimetre ought to be obtained at as low 
a temperature as 500° C. 
The problem we have to face in tlie case of sodium is not quite the same as in the 
case of non-volatile substances such as carbon and platinum. For in this case the 
metal has an appreciable vapour pressure at the temperature at which the experi¬ 
ments are carried out, and part of the conductivity present is doubtless due to the 
spontaneous ionisation of the metal vaponr. A second inconvenience, which is more 
of a practical nature, is caused by the distillation of the metal from the hotter to the 
colder parts of the tube, causing the state of the latter to continually vary. For the 
same reason some of the sodium condenses on the electrode which is supposed to be 
free from it, so that both electrodes emit negative ions. 
We have seen that in the case of platinum and carbon no current was obtained 
when the hot conductor was positively charged; in other words, the conductivity was 
perfectly unipolar. In the case of sodium, owing to the spontaneous ionisation of the 
vapour and the condensation of the metal on the inserted electrode, we should expect 
to get a current in both directions. In the following experiments the first effect must 
have been small, owing to the low vapour-pressure at the temperatures employed, 
while the effect of the second was made small by using an electrode with a very small 
superficial area (a thin platinum wire). It will be seen that in every case the current 
when the sodium surface was negative was more than twenty times its value when 
the surface was positive. 
The apparatus which was used to detect and measure the negative leak from sodium 
will now be described. 
§ 2. Descrii^ion of A 2 yparatus. 
After a great number of trials of various forms of glass apparatus, all of which 
came to an untimely end owing to the joints not l)eing able to stand the continued 
heating or otherwise, the metal apparatus shown diagrammatically in fig. 17 was set 
up. The weldless steel tube ABDC was 76 centims. long and 3’2 centims. in 
diameter, and was kept at zero potential by means of the earth wire shown. The 
straight j^latinum wire A^B, was insulated with sealing-wax at each end and could be 
charged positively or negatively to any desired potential. It formed the electrode 
mentioned above. The whole tube was placed in a small combustion furnace, by 
means of which the central portions could be heated to any desired temperature. The 
