106 NOTICES OF THE MEETINGS [Jan. 238, 
spondent to its length, that the needle is constantly a tangent to 
the line of motion; or, it is that line along which, if a transverse 
wire be moved in either direction, there is no tendency to the 
formation of an electric current in the wire, whilst if moved in 
any other direction there is such a tendency. The direction of 
these lines about and between ordinary magnets is easily repre- 
sented in a general manner by the well known use of iron filings. 
The method of recognizing and taking account of these lines of 
force which is proposed, and was illustrated by experiments during 
the evening, is to collect and measure the electricity set into 
motion in the moying transverse wire; a process entirely different 
in its nature and action to that founded on the use of a magnetic 
needle. That it may be advantageously employed, excellent con- 
ductors are required; and therefore those proceeding from the 
moving wire to the galvanometer were of copper 0.2 of an inch 
in thickness, and as short as was convenient. The galvanometer, 
also, instead of including many hundred convolutions of a long 
fine wire, consisted only of about 48 or 50 inches of such wire 
as that described above, disposed in two double coils about the 
astatic needle: and that used in the careful research contained 
only 20 inches in length of a ‘copper bar 0.2 of an inch square. 
These galyanometers shewed effects 30, 40, or 50 times greater 
than those constructed with fine wire; so abundant is the quantity 
of electricity produced by the intersections of the lines of magnetic 
force, though so Jow in intensity. 
The lines of force already described will, if observed by iron filings 
or a magnetic needle or otherwise, be found to start off from one 
end of a bar magnet, and after describing curves of different magni- 
tudes through the surrounding space, to return to and set on at the 
other end of the magnet; and these forces being regular, it is evident 
that if a ring, a little larger than the magnet, be carried from a 
distance towards the magnet and over one end until it has arrived at 
the equatorial part, it will have intersected once all the external lines 
of force of that magnet. Such rings were soldered on to fitly shaped 
conductors connected with the galvanometer, and the deflections of 
the needle observed for one, two, or more such motions or intersec- 
tions of the lines of force; it was stated that when every precaution 
was taken, and the results at the galvanometer carefully observed, 
the effect there was sensibly proportionate for small or moderate arcs 
to the number of times the loop or ring had passed over the pole. 
In this way, not only could the definite actions of the intersecting 
wire be observed and established, but also one magnet could be 
compared to another, wires of different thickness and of different 
substances could be compared, and also the sections described by 
the wire in its journey could be varied. When the wire was the 
same in length, diameter, and substance, no matter what its course 
was across the lines of force, whether direct or oblique, near to or far 
from the poles of the magnet, the result was the same. 
