LINES OF MAGNETIC FORCE — GALVANOMETER INDICATIONS. 
139 
swing in the two directions is not equal for equal powers, but is greater on one side 
than the other. 
3182. I have not yet seen a galvanometer which has an adjustment for the torsion 
of the suspending filament. Also, there may be other causes, as the presence about 
a room, in its walls and other places, of unknown masses of iron, which may render 
the forces on opposite sides of the instrument zero unequal in a slight degree ; for 
these reasons it is better to make double observations. All the phenomena we have to 
deal with, present effects in two contrary directions. If a loop pass over the pole of 
a magnet (3133.), it produces a swing in one direction ; if it be taken away, the swing 
is in the other direction; if the rectangles and rings to be described (3192.) be 
rotated one way, they produce one current ; if the contrary way, the other and con- 
trary current is produced. I have therefore always in measuring the power of a pole 
or the effect of a revolving intersecting wire made many observations in both direc- 
tions, either alternately or irregularly ; have then ascertained the average of those on 
the one side, and also on the other (which have differed in different cases from sijth 
to s^th part), and have then taken the mean of these averages as the expression of 
the power of the induced electric current, or of the magnetic forces inducing it. 
3183. Care must be taken as to the position of the instrument and apparatus con- 
nected with it, in relation to a fire or sources of different temperatures, that parts 
which can generate thermo-currents may not become warmed or cooled in different 
degrees. The instrument is exceedingly sensible to thermo-electric currents ; the 
accidental falling of a sun-beam upon one of two connecting mercury cups for a few 
moments disturbed the indications and rendered them useless for some time. 
3184. In order to ascertain practically, /. e. experimentally, the comparative value 
of degrees in different parts of the scale or graduation of this instrument and so to 
render it a measurer, the following trials were made. A loop like that before described 
(3133.), fig. 2, was connected with the galvano- Fig- -• 
meter by communications which removed the 
loop 9 feet from the instrument, and it was then 
fixed. A compound bar-magnet consisting of two plates, each 12 inches long, 1 inch 
broad, and 0'5 in thickness, was selected of such strength as to lift a bunch of clean 
iron filings, averaging 45 grains at either extremity. Blocks were arranged at the loop, 
so that this magnet, held in a vertical position, could have one end passed downwards 
through the loop until the latter coincided with the equator of the magnet (3191.) ; 
after which it could be quickly removed and the same operation be repeated at plea- 
sure. When the magnet was thus moved, the loop being unconnected (at one of the 
mercury cups) with the galvanometer, there was no sensible change of place in the 
needles ; the direct influence of the magnet at this distance of 9 feet being too small 
for such an effect. 
3185. It must be well understood, that, in all the observations made with this in- 
strument, the swing is observed and counted as the effect produced, unless otherwise 
expressed. A constant current in an instrument will give a constant and continued 
T 2 
