■i06 Sig. Nobili and Antinori on the Electro-motive Force 



constantly at the surface of mercury, to which the conjoining 

 wires destined to close the circuit are conducted. In the voltaic 

 pile having a certain degree of electric tensio7i, the sparks pass 

 between the zinc and copper poles, either in the case of open- 

 ing or of closing the circuit. In a single Wollaston's element 

 the tension is feeble, and the spark occurs only when the cir- 

 cuit is interrupted. At that moment the current which before 

 was moving, accumulates as it were at the place of interruption, 

 and acquires the intensity necessary to cause the spark. Such 

 tension is wanting in the other case of closing the circuit, and 

 the spark also is absent. 



The currents developed in the electro-dynamic spirals by 

 virtue of magnetism are also in motion, but circulate onlj' for 

 the moment during which they are ap})roaching to or receding 

 fi'om the magnet. It was therefore, we concluded, in one 

 of those two moments that we ought to open the circuit in 

 making the experiment for the spark. 



Thus we arranged our ideas relative to the best disposition 

 of the electro-dynamic spirals : nothing therefore remained 

 but to select a good horse-shoe magnet; to surround the 

 lifter with a copper wire in the maimer before described ; to 

 immerse the extremities of this wire in a cup of mercury, and 

 to raise the one or the other extremity at that precise mo- 

 ment when the lifter was attached to or detached from the 

 magnet. When two persons operate without any kind of 

 machinery, it is more easy to lose than to catch this moment. 

 But when the movements were simultaneous, which happened 

 every now and then, we had the satisfaction of seeing a spark, 

 which left nothing to be desired. 



Such was the mode by which we saw the first spark : but 

 as this beautiful result deserved to be produced at pleasure, 



and a spark is again seen. When the plate and the point are well amalga- 

 mated, the spark will not fail once in a hundred times either at making or 

 breaking contact. I have shown it brilliantly to two or three hundred per- 

 sons at once, and over all parts of the theatre of the Royal Institution. 



As Professor Ritchie expresses it, the spark has not yet been obtained 

 except from a temporary magnet, i. e. from a magnet in the act of being 

 made or destroyed. 1 obtained the first spark from a soft iron magnet 

 made by the well-known influence of electric currents. Sig. Nobili and 

 Antinori obtained the second spark from a soft iron Tiiagnet made so by 

 the influence of a common artificial steel magnet ; their result has been re- 

 peated by a great number of persons. Mr. Forbes of Edinburgh first 

 obtained the spark from a soft iron magnet made so by the influence of 

 the natural loadstone. The latter experiment is also that which I have 

 made with Mr. Daniell's loadstone, lifting only about thirty pounds, and in 

 the manner described. I was not aware of any other modes of performing 

 the experiment except my original one, and Sig. Nobili and Antinori's. 

 — M. F.] 



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