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A New Theory of Terrestrial Magnetism. [April 3 
velocity of rotation around its axis w, and a uniform eledtric 
density a, then there must be an eledtro-magnetic potential 
at any point inside the earth equal to — 
w rcosO, 
and at any point outside the earth— 
4 ^ 1 n 
2 cost/, 
3 r 
where the unit of length is the earth’s radius, r the distance 
from the earth’s centre of the place in question, and 0 its 
co-latitude. 
Now this eledtro-magnetic potential will be accompanied 
with certain magnetic forces varying from point to point, 
and the magnitude of these forces will depend on the interior 
constitution of the earth. As an example of their conclusion 
they examine what would be the distribution of magnetic 
intensity if the earth consisted of a hollow iron shell, and 
they have, using Poisson’s formula, arrived at the following 
result : — If such an iron shell has an initial uniform charge 
of static eledtricity, and if it has an angular velocity of rota- 
tion round a diameter, then, independently of all other 
bodies in the universe, and independently of the coefficient 
of magnetisation of the iron, there will be at any point on the 
surface of the sphere, having a latitude X, a magnetic force 
proportional to — 
{/ 1 + 3 sin a x. 
But this result is the same as that given by Biot’s well- 
known law for the distribution of magnetic intensity on the 
earth’s surface, hence giving considerable probability to their 
theory. 
It is important to notice that in previous examinations of 
the earth’s magnetism it has been usual to start with the 
known law of distribution of magnetic intensity, and then 
deduce what arrangement of magnets, &c., inside the earth 
would lead to this distribution ; but in this new theory of 
Profs. Perry and Ayrton they start merely with an experiment 
described by Prof. Helmholtz, of the effedts of a rotating 
eledtrified body, and show from this that the earth by its 
rotation alone must be magnetic, and next prove that if there 
be an iron shell, thick or thin, in the earth then the distri- 
bution of magnetic intensity on the earth’s surface will be 
the same as is known to exist from observation. 
Next as regards the sign of the eledtric charge on the earth’s 
surface required to produce the earth’s magnetic polarity — 
Is it in accordance with the known phenomena of atmospheric 
