TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 
165 
needle comes to its usual place of rest. But upon repeating the experiment 
of inversion in the direction of the dip (141.), the needle was affected as power- 
fully as before ; the disturbance of the magnetism in the steel magnet, by the 
earth’s inductive force upon it, being thus shown to be nearly, if not quite, 
equal in amount and rapidity to that occurring in soft iron. It is probable 
that in this way magneto-electrical arrangements may become very useful in 
indicating the disturbance of magnetic forces, where other means will not 
apply; for it is not the whole magnetic power which produces the visible effect, 
but only the difference due to the disturbing causes. 
148. These favourable results led me to hope that the direct magneto-elec- 
tric induction of the earth might be rendered sensible ; and I ultimately suc- 
ceeded in obtaining the effect in several ways. When the helix just referred 
to (141. 6.) was placed in the magnetic dip, but without any cylinder of iron 
or steel, and was then inverted, a feeble action at the needle was observed. 
Inverting the helix ten or twelve times, and at such times that the deflecting 
forces exerted by the currents of electricity produced in it should be added 
to the momentum of the needle (39.), the latter was scon made to vibrate 
through an arc of 80° or 90°. Here, therefore, currents of electricity were 
produced by the direct inductive power of the earth’s magnetism, without the 
use of any ferruginous matter, and upon a metal not capable of exhibiting 
any of the ordinary magnetic phenomena. The experiment in everything re- 
presents the effects produced by bringing the same helix to one or both poles 
of any powerful magnet (50.). 
149. Guided by the law already expressed (114.), I expected that all the 
electric phenomena of the revolving metal plate could now be produced with- 
out any other magnet than the earth. The plate so often referred to (85.) was 
therefore fixed so as to rotate in a horizontal plane. The magnetic curves of 
the earth (114. note), i. e. the dip, passes through this plane at angles of about 
70°, which it was expected would be an approximation to perpendicularity, 
quite enough to allow of magneto-electric induction sufficiently powerful to 
produce a current of electricity. 
150. Upon rotation of the plate, the currents ought, according to the law 
(114. 121.), to tend to pass in the direction of the radii, through all parts of the 
plate, either from the centre to the circumference, or from the circumference 
to the centre, as the direction of the rotation of the plate was one way or the 
