[Figure 42] represents the instrument as first constructed by Mr. Saxton, in London.[25] A and B are the ends of the helix, surrounding the cylindrical bar of soft iron between E and F, filling the cavity which has been formed out of the solid iron. The size of bar between the collars E and F, thus formed, is the same as the projections H and G. The wire, a, proceeds from the outside of the coil and makes a suitable contact upon the prong, A, of the magnet: b proceeds from the bottom of the coil, where the winding commenced and makes a similar contact upon the prong, B, of the permanent magnet. One wire extends a little further upon the magnet than the other, so that the shorter one may break its connection sooner than the longer. H and G are projections from the sides of the armature, to which the handle, D, is secured. Let the armature, with its helix, be held up against the ends of the prongs of the permanent magnet; and the wires a and b, in perfect contact with their respective prongs, as shown in the [figure]; if, while in this condition, the keeper is suddenly withdrawn, a spark will appear at the end of the short wire, as it breaks its contact with the prong of the magnet.
Mr. Saxton, however, was still further successful, the following year, in carrying out an idea which occurred to him on the 6th of December, 1832, of producing the same phenomena, with a more convenient and powerful rotating instrument.[26] This new arrangement he was able to test on the 20th of June, 1833, and obtained the spark. On the 22d, he made an unsuccessful attempt, in the presence of Prof. Rogers, of Philadelphia, at the decomposition of water. On the 30th of June he exhibited it at a meeting of the British Association at Cambridge, before Dr. Faraday, Dr. Brewster, Prof. Forbes, Dr. Dalton, and many other distinguished and scientific gentlemen. The experiments made by it were the exhibition of the spark, giving shocks, &c. On the third of July, Mr. Saxton succeeded in decomposing water, by adding a little sulphuric acid, and on the 25th of August, he ignited and melted platinum wire.
Fig. 43.
[Figure 43] exhibits a side view of the instrument: a, a, a, is a compound permanent magnet, consisting of three steel plates, put together, side by side. B and C are two wooden supports, upon the platform A A. To these supports the magnet is permanently secured, by a yoke, S, through which pass two screws into the wooden supports below. M is a cross bar, into which, and at right angles with it, are screwed two arms of round soft iron, R, about five-eighths of an inch in diameter, the whole forming the armature or keeper of the magnet. Upon these two projecting arms, are placed two coils, D′ and D, of copper wire, insulated with silk. The whole is very securely fastened upon the steel spindle, N, which has its journals in the supports, B and B′. On the end of the spindle, N, near the curve of the magnet, there is a small band pulley, F, which is driven by the band or cord of the large wheel, E, and the crank, J. The axis of the large wheel passes through a long socket, L, in the top of the column, H; on one end of the axis the band wheel is fastened and on the other the crank. By this arrangement a very rapid and quiet rotary motion is given to the armature.
In the column, H, there is a socket, into which the stem of the upper part of the column, G, is fitted, which admits of the large wheel being raised or lowered, so as to prevent the band from slipping, and when properly adjusted it is secured by the screw, I. O is an ivory hub, sliding over that part of the spindle immediately projecting beyond the cross bar, M. Upon this ivory hub is a copper disc, C, with a socket, n: b, is a needle made of platinum, which, with its socket, m, is nicely fitted upon the end of the steel spindle, so as to be adjusted to any required angle with the armature, and when adjusted to retain its position. The two ends of the two coils, which leave the centre of the helices, are made to form a contact with the soft iron arms, R R, passing through the coils, D′ and D, thus making the circuit complete with the needle b, upon the end of the spindle, N, by a continuous metallic connection of the arms, with the cross bar, M, and through the cross bar to the spindle, N, in contact with the needle, b. The two ends of the two coils, leaving the outside of the helices, are joined in one, and as they pass through the cross bar, M, are insulated from it, by a piece of ivory, inserted in the cross bar. The united wire then passes into and through the ivory hub, e, forming a perfect contact with the copper disc, c, underneath its socket, n: d, is a cup of mercury, in which the copper disc, c, is always immersed, and the needle, b, twice in every revolution of the armature. The cup, d, is so constructed as to rise and fall, by means of a stem, i, sliding vertically into a socket, e, of its support, and is secured to its position by the screw, h. In this way, its proper height for breaking and closing the circuit may be easily obtained, when the armature is rotating. The proper position for the needle, b, is that in which it is just leaving the mercury, as the keeper arrives at the position, in which its magnetism is neutralized. This position is seen at X, where D″ and D′ are the sides of the coils; c the copper disc; m the cross bar of the armature; R the arm passing through the coil; and b the needle, at that angle which it requires, when the armature is vertical or at its neutral position. It will be observed that the needle is just leaving the mercury, d.
Fig. 44.