LINES OF TERRESTRIAL MAGNETIC FORCE — MOVING RECTANGLES. 
149 
wire gave for twelve revolutions 7°'33, or 0°*61 per revolution. Now the needles of 
the two instruments were not very different in weight and other circumstances, so 
that without pretending to an accurate comparison, we may still perceive an im- 
mense falling-off in both cases, due to the obstruction of the fine wire in the 
Ruhmkorff’s galvanometer; for the thick wire it is from 7°'26 to 0°-138, and for the 
thin wire from 3°'36 to 0°‘6l0. Still the thin wire rectangle has lost far less pro- 
portionately in power than the other; and by this galvanometer is above four times 
greater in effect than the rectangle of thicker wire. Of the thick wire effect less 
than a fiftieth passes the fine wire galvanometer, all the rest is stopped ; of the fine 
wire effect more than ten times this proportion, or between a fourth and a fifth (be- 
cause of the higher intensity of the current), surmounts the obstruction presented by 
the instrument. The quantity of electricity which really passes through the fine wire 
galvanometer is of course far less than in the proportion indicated above. The thick 
wire coil makes at the utmost four convolutions about the needles, whereas in the 
fine wire coil there are probably four hundred or more ; so that the electricity which 
really travels forward as a current, is probably not a hundredth part of that which 
would be required to give an equal deflection in the thick wire galvanometer. Such 
a circumstance does not disturb the considerations with respect to the relative inten- 
sity of the magneto-electric currents from the two rectangles, which have been 
stated above. 
3208, A large square was now constructed of copper wire 0°’2 of an inch in dia- 
meter. The square was 36 inches in the side, and therefore consisted of 12 feet of wire, 
and inclosed an area of 9 square feet ; it was attached to the commutator by ex- 
pedients, which, though sufficient for the present, were not accurate in the adjust- 
ments. It produced a fine effect upon the thick wire galvanometer (3178.) ; for one 
revolution caused a swing deflection of 80^^ or more ; and when its rotation was 
continuous the needles were permanently deflected 40° or 50°. It was very interest- 
ing to see how, when this rectangle commenced its motion from the horizontal plane, 
the current increased in its intensity and then diminished again, the needles showing, 
that whilst the first 10° or 20° of revolution were being passed, there was very little 
power exerted in them ; but that when it was towards, or near the 90°, the power was 
great; the wires then intersecting the lines of force nearly at right angles, and there- 
fore, with an equal velocity, crossing the greatest number in a given time. It was also 
very interesting, by the same indications, to see the two chief impulses (3192.) given 
in one revolution of the rectangle. Being large and massive in proportion to the 
former wires, more time was required for a rotation than before, and the point of 
time or velocity of rotation became more essential. One rotation in a second was as 
much as I could well produce. A speed somewhat less than this was easy, conve- 
nient and quick enough ; it gave fora single revolution near 80°, whilst a revolution 
with one-half or one-third the velocity, or less, gave only 60°, 50°, or even smaller 
amounts of deflection. 
