598 
silver forming an exception; in this case conduction did not become 
perceptible, until the films were several times thicker.') 
J. J. THomson?) discussed the conduction in thin metallic films from 
the point of view of the electron-theory ; but his theory is not in good 
accordance with the observations. W. F. G. Swann ®) tries to find 
an explanation by assuming, that the films are not built up out of 
molecules but of complexes *) of molecules. He gives a mathematical 
discussion of the electric conduction between complexes of that nature, 
starting from special assumptions regarding the mobility of electrons. 
In this manner he succeeds in explaining the sudden rapid increase 
of the specific resistance, as also the negative temperature-coefticient 
of thin films. His theory leads to the conclusion that for very thin 
films Oum’s law would no longer hold. 
In all the above-mentioned investigations films were used which 
had been formed either by chemical methods or by means of cathode- 
discharge. Measurements on the electric resistance of metallic films 
formed by evaporation have not been made so far. Still films obtained 
by that process offer several advantages as compared to those formed 
in a different manner : 
1. The film is easily obtained perfectly uniform °). 
2. According to the kinetic theory, it must be assumed, that a 
metal evaporates as molecules, not as complex groups of molecules; 
this assumption is moreover in accordance with KNUDSEN’s °) measu- 
rements on the maximum rapidity of evaporation of mercury. 
K upsEN °) has further shown, that a molecule which escapes from 
the evaporating metal at the first collision with the wall adheres to 
it, if the temperature of the wall is sufficiently lower than that of 
the evaporating metal; from which it may be inferred, that the thin 
metallic films whieh are formed by evaporation consist of molecules 
and not of molecular complexes *). 
3. The film is formed in a high vacuum, so that there can be 
no danger of any reaction, chemical or otherwise, with gases. 
For these reasons we have carried out the research on the electric 
resistance of thin metallic films which is related in this paper with 
1) Comp. also L. HOULLEVIGUE. Ann. chim. phys. 8. 8. T. 21. p. 197. 19 0. 
2) J. |}. THOMSON. Cambridge Proc. (2) XI. p. 120. 1901. 
3) W. F. G. Swann. Phil. Mag. Vol. 28. Ser. 6. p. 467. 1914. 
4) Comp. also L. HouLLEVIGUE. l.c. 
5) This disposes of the objection raised by Ruwpe (lc), that the large increase 
of specific resistance of thin films might be due to inequalities of the thickness. 
6) M. Knupsen, Ann. der Phys. Bd. 47, p. 697. 1915. 
7) M. Knupsen, Ann. der Phys Bd. 50, p. 472. 1916. 
5) Comp., however, the remark on page 606. 
