136 Mr. 8. Tolver Preston on some Electromagnetic 
Twenty years afterwards, in 1852, in order to test the case 
further, the following experiment was made by Faraday. An 
insulated wire was bent into the form of a rectangle, eda bf 
(fig. 4), and pushed into the slit or interval between two bar- 
magnets of oblong cross section, of which fig. 4 (bis) shows 
the end view. The part of the wire buried between the 
magnets is dotted in the upper figure. The whole was made 
fast to the axis of a convenient rotary apparatus, so that 
the whole could be put in motion about the central magnetic 
axis (common to both magnets). ‘The ends e and f of the 
wire loop were brought out and attached separately to two 
insulated metal rings fastened on the axis of rotation, and 
suitable springs pressed upon these rings, these springs thus 
serving to carry off any current to a conveniently placed 
galvanometer (Phil. Trans. 1852, p. 30, &c.). The whole 
arrangement is accordingly very simple. The magnets with 
rigidly attached rectangular loop were revolved together (as 
one whole) about the central axis, and no current or deflec- 
tion was observed at the galvanometer. The absence of cur- 
rent was explained in the following manner. The outer pro- 
jecting part of the rectangular loop, viz. ad e, was supposed 
by its rotation to cut through the outer lines of force of the 
magnet, which were considered to remain in absolute rest 
while the magnet revolved. The inner part of the rectangular 
loop, viz. ab f (dotted in the diagram), was supposed, on the 
other hand, to cut through the imner or interior magnetic 
lines of force, which were also thought to remain in rigid 
rest, while the (compound) magnet independently revolved 
through them. In this way it was supposed that two oppo- 
site currents tended to be produced in the rectangular loop, 
and so exactly neutralized each other; and the absence of 
current in the loop was thus explained. But instead of 
having recourse to this complicated hypothesis, one can 
explain the absence of current by the simple fact that the 
whole system is at rest when it revolves as a whole, or there 
is no relative motion between its parts, which are rigidly 
connected; and without relative motion there can be no induc- 
tive effect *. 
* Faraday considered that this experiment—where magnet and attached 
wire loop are revolved as one rigid whole—proved that the lines of force 
within the magnet were equal in power to those owtsede the magnet ; 
because he supposed that the lines of force remained in absolute rest 
while the system revolved through them, whereby the current generated 
by the intersection of the internal lines of force was considered equal and 
opposite to that generated by the intersection of the external lines of force. 
Faraday says :—“ The results, when the wire and magnet rotated together, 
