Vol. 8, 1922 
PHYSICS: E. H. HALL 
309 
connection, as a comet shoots by the sun. Accordingly, termination of 
the Hfe of an ion by the taking up of a free electron may be a comparatively 
rare occurrence, most of the ions ending by taking up electrons directly 
from neighboring atoms. 
9. Though the atoms have a regular arrangement in a continuous metal 
crystal, the individual crystals in any ordinary piece of metal are so small 
and so heterogeneously placed with respect to each other, that we may 
think of the atoms and ions as, on the whole, subject to mutual collision 
with any direction whatever of their line of centres. Accordingly we may 
think of the pits forming anywhere on the ions, in one quarter as probably 
as in any other, when no impressed electric field is in operation. 
10. Impressed electric fields of ordinary strength have no appreciable 
influence in determining or preventing the passage of an electron from an 
atom to a neighboring ion.^ Thus a potential gradient of 1 volt per centi- 
meter, imposed from without, would be a very steep one, whereas, if the 
"ionizing potential" of an atom within the solid metal is of the order of 1 
volt, as I believe it to be, the field of force which a neighboring ion may 
exert on an electron is of the order of 10^ volts per centimeter. It is pos- 
sible, however, that when extremely great current densities are used, as in 
attempts to pass beyond the range of Ohm's law, the impressed field may 
be great enough to have a perceptible effect in determining whether an 
electron shall stay on its present atom or pass over to an adjacent ion. 
Such an effect would tend to make the current increase more rapidly than 
the potential gradient and might do something to mask the opposite kind 
of change that would naturally come from the approach of the ions to a 
saturated state of orientation. 
11. The impressed electric field does its work, under ordinary conditions, 
by turning the ions so that the pits upon them, wherever they at first occur, 
move as a positive charge would move, in the conventional direction of the 
field force, while the electron portion of the damaged surface shell of the 
ion moves as a whole in the opposite direction. Thus we have an electric 
current. The effect of the to and fro heat translations of the ion, super- 
posed on the process of orientation, seems to be, practically, to enlarge the 
ion. 
12. The rapidity of the process of orientation of an ion is probably 
controlled by various factors, one of which is, of course, the moment of 
inertia of the ion, but the time occupied in this process is assumed to be 
less than the normal life time of an ion. See (21). 
Ohm's Law 
13. Let the condition of the ion i in figure 1 represent the completed 
orientation due to a field the lines of which run in the direction of F. 
If the atom a, now in contact with i, loses an electron to i, a pit will form 
