LINES OF TERRESTRIAL MAGNETIC FORCE — MOVING RECTANGLES. 
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rectangle may be looked upon simply as conductors ; for as they do not in their 
motion intersect any of the lines of force, so they do not tend to produce any current. 
3193. The apparatus which carries these rectangles, and is also the commutator 
for changing the induced currents, consists of two uprights, fixed on a wooden stand, 
and carrying above a wooden horizontal axle, one end of which is furnished with a 
handle, whilst the other projects, and is shaped as in fig. 4. It may there be seen, 
that two semi-cylindrical plates of copper a h are fixed on the axle, forming a cylinder 
round it, except that they do not touch each other at their edges, which therefore leave 
two lines of separation on opposite sides of the axle. Two strong copper rods, 0’2 of an 
inch in diameter, are fixed to the lower part of the up- 
right c, terminating there in sockets with screws for 
the purpose of receiving the ends of the rods proceed- 
ing from the galvanometer cups (3180.) : in the other 
direction the rods rise up parallel to each other, and 
being perfectly straight, press strongly against the 
curved plates of the commutator on opposite sides : 
the consequence is, that, whenever in the rotation of the axle, the lines of separation 
between the commutator plates arrive at and pass the horizontal plane, their contact 
with these bearing rods is changed, and consequently the direction of the current pro- 
ceeding from these plates to the rods, and so on to the galvanometer, is changed 
also. The other or outer ends of the commutator plates are tinned, for the purpose 
of being connected by soldering to the ends of any rectangle or ring which is to be 
subjected to experiment. 
3194. The rectangle itself is tied on to a slight wooden cross (fig. 5), which has a 
socket on one arm that slides on to and over the part of the wooden axle projecting 
beyond the commutator plates, so that it shall revolve with the axle. Fig, 5. 
A small copper rod forms a continuation of that part of the frame 
which occupies the place of axle, and the end of this rod enters into 
a hole in a separate upright, serving to support and steady the rect- 
angle and its frame. The frames are of two or three sizes, so as to 
receive rectangles of 12 inches in the side, or even larger, up to 36 inches square. The 
rectangle is adjusted in its place, so that it shall be in the horizontal plane when the 
division between the commutator plates is in the same plane, and then its extremities 
are soldered to the two commutator plates, one to each. It is now evident, that 
when dealing with the lines of force of the earth, or any other lines, the axle has only 
to be turned until the upright copper rods touch on each side at the separation of 
the commutator plates, and then the instrument adjusted in position, so that the 
plane of the ring or rectangle is perpendicular to the direction of the lines of force 
which are to be examined, and then any revolution of the commutator and intersect- 
ing wire will produce the maximum current which such wire and such magnetic 
force can produce. The lines of terrestrial magnetic force are inclined at an angle 
