198 
MK. O. W. llICIiARDSOX ON THE ELECTRICAL CONDUCTIVITY 
liyj)<»tliesiH of coiidiictioii in metals Ijy coipnscles wliicli has been elaborated by 
Professors and J. -1. Tiio.MSux.t Acc<jrdiiig to that theory a metal is to be 
regarded as a sponge-like structure of molecules and ccjmparatively large tixed 
positive ions, with small negati\'e ions oi- corpuscles moving freely with great velocity 
throughout the mass. Since the corpuscles do not all leave the metal when they 
strike the surface, it is evident that there must he a surface discontinuity of potential 
wliicli prevents their escape. If now we raise the temperature ot the metal we 
increase the average velocity of the corpuscles, and, provided the energy required to 
take an ion through the surface does not increase with the temperature, many more 
of the i(jus which strike the surface will pass through than before. In this way we 
can calculate the way in which the nnmher ot corpuscles shot oti from unit area ot a 
metal surface varies with the temperature. Idie formula so obtained involves two 
new constants, viz., the number of ions in unit vriimie r)t the metal and the work 
done by an ion in passing thi'ongh the surface. 
It may he permissible to state in anticipation that almost the whole of the 
experimental results are in striking agreement with the theory. In particular the 
theoretical formula makes the saturation current increase enoirnously rapidly with 
the temperature of which it is an exponential lunction. lire experiments show that 
this is actually the case, and the saturation current has been followed over the 
following ranges of values for the three conductors examined : 
For platinum from to 10“'^ ampere per sq. centim. 
,, carlion ,, 1,, 2 ,, ,, ,, 
„ sodium „ 10“^^ „ 2 x 10 “^ ,, total current. 
The corresponding ranges of temperature for platinum and sodium are roughly from 
1000° C. to IG00° C., and from 100° C. to 450° C. respectively. 
Perhaps the most surprising result of the investigation is the relatively enormous 
currents whicli have been obtained. The biggest leak measured was '4 ampere from 
a carbon filament to an electrode placed near it; this corresponded to a current of 
2 amperes per sq. centim. of the carbon surface, the potential on the wire being 
— GO volts. In tliis case the gas pressure was only millim. of mercury, so that 
the ionisation produced by collisions Avas negligible. In all cases care was taken that 
tlie field which was put on the filaments was insufficient either to start a discharge 
oi’ to mainfain one when started. 
The smaller currents wltli sodium were measured by means of a ([uadrant electro¬ 
meter : the largest ('04 ampere) was registered on a Weston ammeter. 
It is evident from these experiments that a metal ii‘ placed in a Amcuum and 
heated to a sufficiently high temperature makes the space around it an extremely 
* ‘Druue’s Ainuileii,’ vol. 1, p. 506. 
t ‘ Ruppoi-ts preseiites an Co.igres Interuatioiial de Rhysiipie,’ Paris, 1000. 
