[Figure 40], represents a top view of the instrument, H is the platform; C the disc; a the segment; A the permanent magnet; J the wire attached to the axis, P; G and G are the two standards. E the crank; and I the wire attached to the segment a.
Mr. Saxton,[23] in a letter to Mr. Lukens, dated, London, April 14th, 1832, after describing Dr. Faraday’s rotating disc, figures [39] and [40], says, “I have made this experiment in a different way, and succeeded satisfactorily. The method was as follows: A coil of wire wrapped with silk, similar to that used in the galvanometer, was attached, by the ends, to the wires of the galvanometer. On passing this roll, backward and forward, upon one of the poles of a horse-shoe (permanent) magnet, or placing it upon and removing it from either pole, I have made the needle of the galvanometer to spin round rapidly.” [Figure 41], represents Mr. Saxton’s plan.
Fig. 41.
N and S represent the north and south poles of the horse-shoe permanent magnet. C is the coil of wire, wound round a spool of an oblong shape, through the centre of which there is an opening sufficiently large to admit either of the prongs of the magnet through it. A and B are the ends of the wire leaving the coil, and are connected with the galvanometer.
Mr. Saxton on the 2d of May, 1832, obtained the spark by the following arrangement of the permanent magnet and the helix of wire round the armature. In relation to this instrument, he thus writes to Mr. Lukens, of Philadelphia, dated, London, May 11th, 1832. Jour. Frank. Int. vol. 13, p. 67. “Since my last I have heard of a method of producing a spark from a magnet, discovered I think by an Italian.[24] This experiment I made at once upon a large horse-shoe magnet, which I am making for Mr. Perkins and his partners. One of your large magnets will answer the same purpose. Make a cylinder of soft iron of an inch, or three-fourths of an inch, in diameter, and of the usual length of the keeper; place two discs of brass or wood upon this cylinder, and at such a distance apart that they will conveniently pass between the poles of the magnet; between these wind, say fifty feet of bobbin wire, which may be of iron covered with cotton; let the ends of this coil be bent over the ends of the cylinder and brought down until they touch the poles of the magnet. The ends should be of such a length, that on bringing the cylinder to the magnet, one of the ends will touch, when the cylinder is about half an inch from the magnet, and the other at one-fourth of an inch. The cylinder being thus arranged, and in contact with the magnet, on drawing it suddenly away a spark will pass between the end of the wire, and the pole of the magnet.”
Fig. 42.