120 



Progress in Physics. 



[January, 



Thompson's Syphon Recorder. — Among the inventions of Sir William 

 Thompson, is one, which has recently come into use for registering feeble 

 indications, made by weak currents of electricity, such currents, in fact, as are 

 used in working Atlantic cables. 



The principle of the invention may be explained by the aid of Figs. 24, 25, 

 and 26. In Fig. 24, n s are the poles of a powerful permanent magnet, capable 



Fig. 24. Fig. 25. 



Fig. 26. 



of lifting several pounds. To still further increase the power of the magnetic 

 field Mr. Varley's plan of placing a piece of soft iron, s n, between the poles 

 without touching them is adopted. a b d e is a very light coil of a small 

 number of turns of fine wire, through which the feeble current from the line- 

 wire passes. This coil is kept stiff by being stretched over pieces of any light 

 stiff material, like the boom in the rigging of a ship. By 

 this arrangement, whenever the current from the line passes 

 through the coil, the coil will move in or out according to the 

 direction of the current. 



Fig. 25 is a diagram of the same arrangement viewed 

 from above, n s being the poles of the magnet, and b the 

 coil. A silk thread, d, connects one side of the coil with 

 one of the bends of the syphon, e, so that as the coil, b, 

 moves to and fro, the leg of the syphon, e, moves to and fro 

 near the surface of the slip of paper, r, which is drawn by 

 clockwork over the brass roller, f f. This roller is insulated 

 with vulcanite supports, and is connected with an electrical 

 machine, in consequence of which the liquid or ink in the 

 capillary syphon made of glass spirts out against the paper, 

 and thus the motions of the syphon and coil are recorded. 

 Fig. 26 is another view of the same apparatus, A being 

 the vessel containing the ink, d the syphon, e the brass roller, and f f 

 the paper. 



New Experiments in Diamagnetism. — The following experiments were made 

 with a powerful electro-magnet belonging to Lord Lindsay ; the iron horse- 

 shoe is four inches in diameter, and the bent bar is seven feet long. The two 

 helices are of copper wire nearly a quarter of an inch in thickness, and a very 

 powerful Grove's battery is employed to excite the magnet. The experiments 

 hereinafter described were devised by Mr. C. F. Varley. 



Some rings were prepared, consisting of pieces of bent copper wire, soldered 

 together at the ends; the wire was T V inch in diameter, and the ring i£ inch. 

 When the ring, A, Fig. 27, was inserted between the movable poles, n s, of the 

 electro-magnet, each pole 2 inches square, the ring fell gradually and slowly until 

 the upper part of the ring, c, was level with a, or about T \j of an inch below the 

 upper surfaces of the two poles. It then fell rapidly until the lower portion 

 of the ring arrived at b, when it commenced to fall slowly once more, and soon 

 afterwards it passed out of the magnetic field, when it fell at the ordinary rate. 

 While the ring was going into the magnetic field it experienced great resistance 

 to its fall ; the moment it passed into a sensibly uniform magnetic field, it fell 



