Experiments upon the Induction of Metallic Coils. 309 
twelve feet long, covered with silk. The most delicate instru- 
ments are those constructed upon the principle described by Mr. 
Locke in the thirteenth volume of this Journal, in which a small wire 
is wound round a disk, so that the needle may be near the electrical 
current at every angle of deviation, and the needle is suspended by 
asilk thread. But all instruments of this size are galvanoscopes, 
merely indicating the presence of a small current, without giving us 
the power of comparing with each other tbe force of two currents, 
as exhibited in different experiments. For this purpose a large in-~ 
strument constructed upon the plan mentioned above will be found 
very useful and correct. I have used a galvanometer composed of 
a needle two feet long, suspended upon a pivot, and surrounded 
by a coil of zinc ribbon covered with silk. This instrument will be 
found to be much more powerfully affected than those constructed 
with a coil of wire, and many very interesting and useful experi- 
ments may be made with it. 
The effect of a coil of metallic ribbon has never been satisfacto- 
rily explained. Analogy would lead us to suppose that a current 
passing through a conductor in one direction would give rise to a 
current in an opposite direction, in an adjacent wire, from the known 
effects of electric and magnetic induction. But this appears not to 
be the fact, as two currents running in the same direction, increase 
the effect of each other, by being brought 
nearly together. Thus in the coil a be (fig. 1.) 
the current passes in the same direction through 
the adjoining layers a, 5, ¢, and the effect of 
the current in a, upon the current in }, instead 
of retarding its motion and diminishing its ef- 
fect will be greatly to increase its intensity. 
And as every successive layer of the coil will 
produce the same effect, they all conspire to increase this intensity. 
That the effect of the coil is not entirely owing to induction of any 
kind, is proved by the experiment that a current from a calorimotor 
passed through a long wire or metallic ribbon, gives a spark, even 
when the greatest care is taken to prevent any portions of the con- 
ductor from lying near each other. This spark will not be as great 
as when the conductor is coiled in the form of a flat spiral, but the 
fact of its giving a spark, while a shoft conductor will not give any, 
shews that it is partly owing to the distance which the electric fluid 
is obliged to travel. The following experiment also shews that the 
Fig. 1. 
a 
