1879*] A New Theory of Terrestrial Magnetism. 
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IV. A NEW THEORY OF TERRESTRIAL 
MAGNETISM. 
By Profs. Perry and Ayrton. 
S T the meeting of the Physical Society on the 7th inst. 
Mr. Ayrton explained, on behalf of Prof. Perry and 
himself, a theory of terrestrial magnetism which has 
the great novelty that it makes the existence of the earth’s 
magnetism depend solely on the earth’s daily rotation, and 
does not require, as do all other theories based on electro- 
magnetic phenomena, the existence of other bodies in the 
universe. In faCt, they have arrived at the result that if 
any body of any material has a static eleCtric charge, and if 
it rotates about an axis, then per se there will be a magnetic 
field in the interior of this body as well as in the neighbour- 
hood outside. 
In 1876 Mr. Rowland, working in Professor Helmholtz’s 
laboratory, proved experimentally that a quantity of electri- 
city in mechanical motion aCted like an eleCtric current in 
deflecting a magnet, and it is on this result that Profs. Perry 
and Ayrton have based their whole theory. For they point 
out that since the points near the surface of the earth have 
different linear velocities from those in the interior (although 
all the points have the same angular velocity of rotation 
round the earth’s axis), it follows that if the earth had an 
initial electrical charge, residing of course in accordance with 
the well-known electrical law, on its surface, the electrified 
particles would have velocities relative to the remainder ; 
hence, as a direCt consequence of the results of the experi- 
ments published by Professor Helmholtz, the interior of the 
earth would be a magnetic field, quite independently of its 
interior constitution, and precisely similar reasoning, of 
course, proves that outside the earth’s surface there would 
also be a magnetic field. 
In order to solve the difficult problem of determining the 
eleCtro-magnetic potential at any place, they have calculated 
the magnetic forces produced at any point, first by the rota- 
tion of a small electrified area on the surface of the earth, 
and then, by summation, the force produced by the rotation 
of the whole electrified surface ; and they have shown that 
these forces are the same as would be produced by certain defi- 
nite distributions of attracting matter over the earth’s surface, 
from which, by the use of spherical harmonics, they have 
arrived at the conclusion that if the earth had an angulaj 
VOL. ix. (n.s.) u 
