607 
. dN@ 5 Dre ° 
will then be ET and that of collisions with the walls 2 x 4NQ. 
As the walls consist partly of molecules and partly of free spaces 
where forces may act on them, we will put the number of colli- 
sions with the walls equal to f/x 2x4 NQ. The total number 
: a ; 2 ak / 
of collisions per second and per electron is thus = 2( + + }, 
2 2d 
Hence the mean free path of the electron in the film will be 
2 
EER 
Jeen 
a 2d 
Substituting this value in the formula given by the electronic 
theory we find the electric conductivity of the film equal to 
Ne? (32 
Be aE a 
4ul (1 + | 
whereas for the metal in thick layers the expression 
Ne? £22 
em == rm 
4a 1 
holds. Hence 
1 
KH Fy Swe, 
ne 
ia 2d 
1 1 it 
G Gp ra fa 
2d 
(6 — On) d = 5 2 Om — constant 
which actually corresponds to a hyperbola. This simple reasoning 
naturally only holds, as long as 4 is not too large with respect to d. 
Finally some remarks may be made on the change of the resist- 
ance of thin metallic films with the time. As mentioned above, the 
change of the resistance of the films was negligible in our experi- 
ments; this is true for the temperature at which our experiments 
were made (room-temperature, about 22° C.). The matter becomes 
different, if the films are heated; we have investigated the behaviour 
of the films in heating more especially for platinum. 
For comparatively thick films the change of the resistance on 
heating is not very great and after some time the resistance reass- 
mes a constant value; it was thus possible for films of about 
