TO THE MOVEMENT OF MACHINES. 529 



tion 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 be- 

 ginning 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 ter- 

 minated 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 wi receded. The contrarj^ effect might be observed on 

 removing the armature by the blows of a hammer. 



21. 



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 cir- 

 cuit 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 Avhen the circuit was 

 broken by the separation of the wires ab or cd; 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 interme- 

 diary 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 tlicse same circumstances on the 



