144 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIX.) 
of 69° to the horizontal plane. As, however, only comparative results were required, 
the instrument was, in all the ensuing experiments, placed in the horizontal plane, 
with the axis of rotation perpendicular to the plane of the magnetic meridian ; under 
which circumstances no cause of error or variation was introduced into the results. 
As no extra magnet was employed, the commutator was placed within 3 feet of 
the galvanometer, so that two pieces of copper wire 3 feet long and 0-2 of an inch in 
thickness, sufficed to complete the communication. One end of each of these dipped 
into the galvanometer mercury cups, the other ends were tinned, amalgamated, 
introduced into the sockets of the commutator rods (3193.), and secured by the 
pinching screw (fig. 4). 
3195. When a given length of wire is to be disposed of in the form best suited to 
produce the maximum effect, then the circumstances to be considered are contrary 
for the case of a loop to be employed with a small magnet (39. 3184.), and a rect- 
angle or other formed loop to be employed with the lines of terrestrial force. In the 
case of the small magnet, all the lines of force belonging to it are inclosed by the 
loop ; and if the wire is so long that it can be formed into a loop of two or more 
convolutions, and yet pass over the pole, then twice or many times the electricity 
will be evolved that a single loop can produce (36.). In the case of the earth’s 
force, the contrary result is true ; for as in circles, squares, similar rectangles, &c. 
the areas inclosed are as the squares of the periphery, and the lines of force inter- 
sected are as the areas, it is much better to arrange a given wire in one simple 
circuit than in two or more convolutions. Twelve feet of wire in one square inter- 
sects in one revolution the lines of force passing through an area of nine square feet, 
whilst if arranged in a triple circuit, about a square of one foot area, it will only in- 
tersect the lines due to that area ; and it is thrice as advantageous to intersect the 
lines within nine square feet once, as it is to intersect those of one square foot three 
times. 
3196. A square was prepared, containing 4 feet in length of copper wire 0’05 of 
an inch in diameter ; it inclosed one square foot of area, and was mounted on the 
commutator and connected in the manner already described (3194.). Six revolutions 
of it produced a swing deflection of 14° or 15°, and twelve quick revolutions were 
possible within the required time (3104.). The results of quick and slow revolutions 
were first compared. Six slow revolutions gave as the average of several experiments 
15°'5 swing. Six moderate revolutions gave also an average of 15°'5 ; six quick 
revolutions gave an average of 15°*66. At another time twelve moderate revolutions 
gave an average of 28°-75, and twelve quick revolutions gave an average of 31°'33 
swing. As before explained (3186.), the probable reason why the quick revolutions gave 
a larger result than the moderate or slow revolutions is, that in slow time the later 
revolutions are performed at a period when the needle is so far from parallel with the 
copper coil of the galvanometer, that the impulses due to them are less effectually 
exerted. Hence a small or moderate number of revolutions and a quick motion is 
