Prof. Challis on a Theory of Magnetie Force. 99; 
induction of magnetism will be greater, the more directly the 
primary stream flows through the bar, as is known to be the case 
by experiment. 
26. When a coil of wire in the form of a helix surrounds a 
cylinder of soft iron, and a galvanic current is sent through the 
wire, magnetism is found to be induced in the iron. This fact 
is explained by the theory as follows. The turns of the helix 
being supposed to be very little apart, each turn will be nearly 
in a plane perpendicular to the axis of the cylinder. Hence the 
circular motions, which, according to the theory of galvanism, 
take place about the wire, will be very nearly in planes passing 
through that axis. Suppose the course of the current to be 
from the end B towards the end A of the cylinder, and the 
turns of the helix to proceed from the left hand, over the ecylin- 
der, to the right hand of a person looking from B towards A. 
Then, since the circular motion about any portion of an elec- 
trode in which the current is conceived to flow parallel to the 
earth’s axis from south to north is always in the direction of 
the earth’s rotation, it follows, in the case supposed, that if the 
circular motion within the cylinder be resolved into parts par- 
allel and perpendicular to its axis, the former will all be in the 
direction from B towards A, and the latter will very nearly 
destroy each other. Thus the result will be a longitudinal stream 
from B towards A, the velocity of which, if the helix be pretty 
close to the cylinder, will be greatest along its axis. The ex- 
planation of the manner in which this stream induces mag- 
netism in the iron core, is precisely the same as that applied in 
the preceding article to induction by terrestrial magnetism. The 
pole A of the magnetized cylinder corresponds to that pole of a 
magnetic needle which points northward. The contrary would 
plainly be the case if the helix, instead of being dextrorsum, as 
supposed above, had been sinistrorsum. All these results agree 
with well known experimental facts. 
27. The theory I now proceed to give of terrestrial magnetism 
rests on no other hypotheses than those on which the general 
physical theory is based. It will be assumed that the earth and 
all the bodies of the solar system are composed of spherical 
inert atoms, of different magnitudes, and in different states of 
aggregation, and that the ether pervading their interiors is in 
the same state of density as in the external spaces. Any in- 
vestigation of the motions impressed on the ether by the known 
motions of the bodies of the solar system, must, in order to be 
consistent with the whole preceding argument, set out from these 
hypotheses. The following considerations will, I think, show 
that they lead to conclusions which are in accordance with ob- 
served facts of terrestrial and cosmical magnetism. 
H 2 
