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 

 a silk 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 the 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 ab c (fig. 1.) 

 the current passes in the same direction through 

 the adjoining layers a, b, c, and the effect of 

 the current in a, upon the current in b, 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 short 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 



