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Proceedings of the Royal Society of Edinburgh. [Sess. 
the effect of a transverse field was to increase the resistance. But in 
nickel the resistance is diminished in a transverse field. To bring the 
electron theory into accord with fact, Professor E. P. Adams (8) has 
modified Sir J. J. Thomson’s theory by introducing a term depending on 
the change of molecular configuration, and has obtained an expression 
which may he either positive or negative according to the relative values 
of the different terms. 
The following brief summary of his original theory was given by Sir 
J. J. Thomson in the Proceedings of the Cambridge Philosophical 
Society (9) : — 
“ According to this theory, the current through a metal is carried by 
means of corpuscles, those small negatively electrified particles which 
constitute the cathode rays, which are given off by incandescent metals and 
also by metals when exposed to ultra-violet light. These corpuscles are 
assumed to be distributed throughout the volume of all metals, being 
produced by the corpuscular dissociation of the molecules. These particles, 
like the particles of a gas, are supposed to be moving rapidly in all 
directions, their kinetic energy, like that of the molecules of a gas, being 
proportional to the absolute temperature. Under the action of an electric 
field these charged corpuscles acquire a drift in a definite direction — the 
opposite direction to the electric force, since their charge is negative. This 
drift of the corpuscles under the electric field constitutes the current 
through the metal. If n is the number of corpuscles per unit volume of 
the metal, u the velocity of drift in the negative direction of x, e the 
charge on a corpuscle, then the intensity of the current parallel to the 
axis of x is equal to neu. If X is the electric force, m the mass of a 
molecule, t the average time between two collisions of a corpuscle, u is 
equal to tX.e/2m ; if X is the mean free path, c the velocity of mean square, 
t = \/c‘, thus” 
the current = X - -=Xx conductance, 
2 me 
and the resistance has the value 2 mc/(ne 2 \). 
Of the quantities involved in this expression, the mean free path X is 
the one which seems most likely to be influenced by an applied magnetic 
force. As worked out in the papers already referred to, the explanation 
depends on the length of the mean free path or the average time of free 
motion from encounter to encounter of the negative corpuscles with the 
neutral molecules of the material. 
The theory so far as it goes is obviously incomplete. If it cannot in 
an obvious manner explain the general nature of the phenomenon, it can 
