LINES OF TERRESTRIAL MAGNETIC FORCE — MOVING RECTANGLES. 147 
3202. At another time I compared the effect of equable revolutions with other re- 
volutions very irregular in their rates, the motion being sometimes even backwards 
and continually differing in degree by fits and starts, yet always so that within the 
proper time a certain number of revolutions should have been completed. The rect- 
angle was of wire 0’2 of an inch thick ; the mean of many experiments, which were 
closely alike in their results, gave for two smooth, equable revolutions 17°-5, and also 
for two irregular uncertain revolutions the same amount of 17°'5. 
3203. The relation of the current produced to the mass of the wire was then exa- 
mined ; a relation, which has been investigated on a former occasion by loops and small 
magnets (3133.)*. For the present purpose two other equal squares were prepared, 
each a foot in the side, but the copper wire of which they consisted was respectively 
OT and 0‘2 of an inch in diameter; so that with the former rectangle they formed a 
series of three, having the same size, shape and area, but the masses of the moving 
wire increasing in the proportion of one, four and sixteen. When the rectangle of 
OT wire was employed, six direct revolutions gave an average result of 41°-75, and 
six to the left gave 46°’25 ; the mean of the two is 44°, and this divided by 6 gives 
7°'33 as the deflection per revolution. Again, three direct revolutions gave 20°T2, 
and three reverse revolutions 23°T ; the mean being 21°'61, and the deflection per 
revolution 7°’20. This is very close to the former result with six revolutions, namely 
7°'33, and is a large increase upon the effect of the rectangle of wire 0’05 in diameter, 
namely 2°'61 ; nevertheless, it is not as 4 : 1 ; nor could such a result be expected, in- 
asmuch as the mass of the chief conductor remained the same (3137.). When the 
results are compared with those made with like wires in the form of loops, they are 
found to be exceedingly close ; in that case the results were as 16° to 44°'4 (3136.), 
which would accord with a ratio in the present case of 2°'61 to 7°'26 ; and it is as 
2°’61 to 7°’242, almost identical. 
3204. The average of the direct and reverse revolutions is seen above to differ con- 
siderably, i. e. up to 4° and 5° in the higher case. This does not indicate any error 
in principle, but results simply from the circumstance, that when the needles were 
quiescent in the galvanometer, they stood a little on one side of zero (3182.). I did 
not wish to adjust the instrument at the time, as I was watching for spontaneous 
alterations of the zero place, and prefer giving the numbers as they came out in 
the graduation, to any pen-and-ink correction of the notes. 
3205. The third square of 0'2 wire gave such large swings, that I employed only 
a small number of revolutions. Three direct revolutions gave an average of 25°‘58 ; 
three reverse revolutions gave 28°'5 ; the mean is 27°‘04, and the amount per revolu- 
tion 9°-01. Again, two direct revolutions gave 17°'5 ; two reverse revolutions gave 
18° ; the mean is 17°‘75, and the amount per revolution 8°’87 ; the mean of the two final 
* See a corresponding investigation by Christie. Philosophical Transactions, 1833, p. 120. 
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