158 DOVE ON THE ELECTRICITY OF INDUCTION. 



of Ampere, that two currents cutting each other are mutually 

 attracted when both flow from the angle formed by their intersec- 

 tion or flow towards it, but, on the contrary, they repel each other 

 when the one flows towards, the other from that angle. Now, if 

 one of these circles is a fixed wire-ring, in the coils of which the 

 current of a galvanic battery is circulating, the other a closed 

 wire-ring of somewhat larger diameter capable of revolving round 

 the first, it will easily be perceived, that for every whole revolution 

 of this ring round the first, two alternating currents of equal in- 

 tensity will be induced. This ring, placed upon the axis of a 

 Saxton's machine, forms a corresponding apparatus of induction, 

 which is however of an entirely electro-dynamic nature*. In the 

 absence of such an apparatus, I have employed the second mode 

 of electro-dynamic induction only, in which an incipient current, 

 or one just on the point of ceasing, acts upon a secondary wire 

 at rest. 



One of Grove's platino-zinc elements was closed by a spiral A 

 of thick copper wire. A second spiral B of thin copper wire 

 400' long surrounded the first spiral, and was itself connected 

 with a third spiral C 400' long, and of the same thickness of 

 wire. This third spiral C was inserted and insulated in a fourth 

 spiral D 400' long, which could be closed by handles, or some 

 other method of testing the current. The galvanic current cir- 

 culating in the connecting wire A induced in the first instance a 

 secondary current in B, which, traversing C, produced a current 

 of the third order in D. The changes were now examined which 

 took place in the current of the third order, when solid iron 

 cylinders, or bundles of iron wires enclosed in entire or cut 

 tubes, were inserted into the spiral C. The arrangement was 

 made in the same manner when the primary current was that 

 produced by the discharge of an electric battery. A flat copper 

 spiral A imbedded in resin, 11^ inches in diameter, and consist- 

 ing of 31 coils of copper wire 53^ feet long and § line in thick- 

 ness, placed upon an insulating glass foot, formed a part of the 

 connecting circuit of the battery. Opposite to this, and only 

 separated fi'om it by a plate of glass or of mica, was a second 

 flat spiral, the coils of which were exactly parallel to those of 



* Instead of the apparatus liere described, that proposed by Henry may 

 be employed, in which the empty keeper of the Saxton's machine, instead of 

 rotating before a magnet, rotates in front of two equal coils of wire, through 

 whicli a galvanic current is circulating. 



