Page:Scientific Memoirs, Vol. 1 (1837).djvu/541

Rh is not in perfect equilibrium, being stronger on one side than on the other, there will be a deviation of the needle proportional to the difference of the currents which traverse the wires $$a\, b,\, c\, d$$. This agrees with the experiments which Mr. Faraday has related at the beginning of his Eighth Series, on the subject of decompositions produced by a single pair of voltaic plates. In short what is termed tension is the effect of forces equal and contrary in direction. In mechanics such forces destroy themselves, their sum being zero; but in physics it is different.

With regard to the direction of the magneto-electric current which occasions the shock, it is the same as that of the voltaic current. This was proved by a galvanometer, the multiplying wire of which terminated at the points $$e,\, f$$. There was a deviation on a part of the voltaic current traversing the secondary branch $$e,\, f$$. On applying the armature, the needle of this galvanometer advanced, at the same time that the needle $$m$$ receded. The contrary effect might be observed on removing the armature by the blows of a hammer.

The following are some further experiments relative to this subject. The extremities of the bar were surrounded with a thin plate of copper, fig. 6, in the circuit of which was placed a galvanometer. On applying the armature, the needle was unaffected by it; but after having wound the ends of the multiplying wire around the points $$e,\, f$$, and the circuit being thus closed, a considerable deviation took place.

An analogous result is shown in the following experiment. On plunging two thin plates of copper, held firmly in the hands, in the cups $$a,\, b$$, or $$c,\, d$$, of the bar, fig. 4, there was no shock when the circuit was broken by the separation of the wires $$a\, b$$ or $$c\, d$$; for the human body formed part of a circuit, in which equal excitations took place on two opposite sides. The thin plates being plunged into the cups $$c$$ and $$b$$, a violent action took place at the instant of disjunction.

I formed a thermo-electric circuit of bismuth and antimony, in which was interposed a galvanometer: after having heated the two solderings to the same degree, there was no deviation of the needle; but the multiplying wire having been placed so as to form an intermediary branch, and the solderings being on opposite sides, there was a considerable deviation. This would not have taken place if the circuit of bismuth and antimony had been in its normal state, for then it would have had to conduct the greatest part of the thermo-electric current, provided that the multiplying wire was sufficiently long and thin to intercept only an extremely feeble part of it.

It seems to me that there are circumstances which cause metals to lose their conducting power, and that these same circumstances on the