LINES OF MAGNETIC FORCE — MOVING RECTANGLES. 
151 
3212. The moving wire, in place of being formed into a, rectangle, may be adjusted 
as a ring ; and then the advantage is obtained of the largest area which a given 
length of wire can inclose, and therefore for a uniform wire, the obstruction to the 
induced current, as respects its conduction, is the least. Small rings of one or several 
convolutions will probably be very valuable in the examination of small and local 
magnets under different circumstances. One consisting of ten spirals of copper wire 
0°’032 of an inch in diameter, containing 49 inches, in a ring about T5 inch in dia- 
meter, gave but small results under the earth’s influence ; but when brought near a 
horseshoe magnet told, in its effects for every difference in distance or in position. A 
single ring 4 inches in diameter, being made of a convolution of copper wire 0'2 in 
thickness, was employed with the earth’s magnetic force as before ; it gave as the 
average of six revolutions many times repeated 5°‘995, or 0°'999 per revolution. For 
twelve revolutions it gave a mean of 12°’37«5, or 1°'031 per revolution*; the mean of 
the two results with such different numbers of revolutions being 1°, Another ring, 
consisting of twenty-six convolutions of copper wire 0’04 of an inch in diameter, was 
constructed and had a mean diameter of 3’6 or 3'7 inches ; it contained 300 inches 
in length of wire. So the masses of the metal in the two rings are nearly the same, 
but the latter wire is singly only -^th of the mass of the former. It gave for twelve 
revolutions a mean of 6°'25, or 0°’52 per revolution. With the earth’s power and the 
thick wire galvanometer, it gave therefore little more than half the result of the single 
thick wire ring. We know from former considerations (3206.), that if the 300 inches 
had been made into one single ring, it would have given a very high effect compared 
to the present. 
3213. The application of the principle of the moving wire in the form of a revolving 
rectangle, makes the investigation of conducting power, and the results produced by 
difference in the nature of the substance, or in diameter, i. e. mass, or in length, very 
easy ; and the obstruction offered by those parts, which moving not across but 
parallel to the lines of force (3071.), have no exciting action but perform the part of 
conductors merely, might be greatly removed by making them massive. They might 
be made to shift upon the axle so as to bear adjustment for different lengths of wires, 
and the commutator might in fact be made to a large extent a general instrument. 
3214. In looking forward to further applications of the principle of the moving 
wire, it does not seem at all unlikely that by increased delicacy and perfection of 
the instrument, by increased velocity, by continued motion for a time in one direc- 
tion and then reversal of the revolution with the reversal of the direction of the 
swing, &c., it may be applied with advantage hereafter to the investigation of the 
earth’s magnetic force in different latitudes and places. To obtain the maximum 
* sin Ap^=sin2-9975 = sin2 59-85— -0522925 
sin^l^=sin6-1875 = sin6 ll-25 = -1077825 '-i^^^ = -0538912. 
