150 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIX.) 
3209. Observations were now made on the measurement of one rotation having- an 
easy quick velocity. The average of fifteen observations to the right, which came 
very near to each other, w'as 78°'846 ; the average of seventeen similar observations 
to the left was 7S°'382 ; and the mean of these results, or 78°'614, I believe to be a 
good first expression for this rectangle. On measuring the distances across after this 
result, I found that in one direction, i. e. across between the intersecting portions of 
wire, it was rather less than 36 inches; having therefore corrected this error, I re- 
peated the observations and obtained the result of 8]°'44. The difterence of 2°‘83, I 
believe to be a true result of the alteration and increase of the area on making it 
more accurately 9 square feet ; and it is to me an evidence of the sensibility and 
certainty of the instrument. 
3210. As the two impulses upon the needles in one revolution (3208.) are here 
sensibly apart in time, and as the needle has as evidently and necessarily left its first 
place before the second impulse is impressed upon it, so, that second impulse cannot 
be so effectual as the first. I therefore observed the results with half a revolution, 
and obtained a mean of 41°'37 for the effect. This number evidently belongs to the 
first of the two impulses of one revolution ; and if we subtract it from 81°'44, it gives 
40°‘07 as the value of the second impulse under the changed place of the needle. 
This difference of the two impulses of one revolution, namely 41°’37 and 40°‘07, is in 
perfect accordance with the results that were to be expected. 
3211. The square of this same copper wire, 0'2 in thickness, employed on a former 
occasion (3205.), had an area of one square foot, so that then the lines of force affected 
or affecting the moving wire, were one-ninth part of what they are in the present 
case: the effect then was 8°‘94 per revolution. If, in comparing these cases, we take 
the ninth part of 81°'44, it gives 9°‘04 ; a number so near the former, that we may 
consider the two rectangles as proving the same result, and at the same time the 
truth of the statement, that the magneto-electric current evolved is as the amount of 
lines of force intersected. A ninth part of the result with the large rectangle (78°'614), 
before its area was corrected, is 8°’734 ; so that the one is above and the other below 
the amount of the 12-inch rectangle. As that was not very carefully adjusted, nor 
indeed any of the arrangements made as yet with extreme accuracy, I have little 
doubt that with accurately adjusted rectangles the results would be strictly pro- 
portional to the areas*. 
* The 9square feet rectangle gave 81°‘44 sin?i-^=sin40‘72=sm40° 43''2='6523630 : or taking 41°-37 
for the half revolution for - A (3210.) sin41°'37=sin41° 22'’2= '6609190, which dmded by nine give '073435 
as the force per square foot. The 1 square foot rectangle of like wire (3205.) gave '07714, or '07793 as the 
force of one revolution ; the first of which is '00370 more than i of the measure of the effect of the large square ; 
the difference being about of '07714, or the whole force of one revolution. 
