4*8 Mr. W. Sturgeon on the Theory of Magnetic Electricity. 



placed on a pivot either above or below, with a slight direc- 

 tive tendency in the plane of the plate, will, with a very mo- 

 derate uniform velocity of the latter, become steadily deflected 

 at right angles to the edge or plane of the revolving disc. 

 Indeed, the needle, although at nearly two inches distant from 

 the edge, is very frequently thrown several times round on its 

 pivot by a sudden motion of the disc. 



The line of greatest energy in the area of the disc by the 

 arrangement in fig. 10, is in that diameter which joins the mag- 

 netic poles ; and its general tendency is in the direction of the 

 straight arrow, but becomes inverted by inverting the motion 

 of the plate. When one magnet only is employed, as in fig. 16 

 and 17 (Plate I. vol. i.), the line of greatest energy in the area 

 of the disc is always a curve, unless the plate be very small. 



By looking over Mr. Faraday's paper, I find that amongst 

 other ingenious arrangements, he has also employed a disc of 

 copper in some of his very interesting experiments ; but the 

 arrangements with that gentleman's apparatus are very diffe- 

 rent to those of mine, already described. 



Mr. Faraday has given to one of his revolving discs the title 

 of " a new electrical machine ;" and as the deflections which 

 he obtained by this apparatus were by the employment of a 

 delicate multiplying galvanometer, and those which I have 

 described were obtained by a heavy needle, without any mul- 

 tiplying apparatus whatever, it may perhaps be interesting to 

 some readers, if we were to bring into one view the results 

 obtained by Mr. Faraday's " new electrical machine," and those 

 which I have shown to be produced by my comparatively old 

 one. 



Besides the delicate multiplying galvanometer which Mr. Fa- 

 raday has described, he also states that he employed, what I 

 believe to be the largest artificial magnet in the world, — the 

 compound magnet belonging to the Royal Society of London ; 

 " composed of about 450 bar magnets, each fifteen inches long, 

 one inch wide, and half an inch thick, arranged in a box so as 

 to present at one of its extremities two external poles. These 

 poles projected horizontally six inches from the box, and were 

 each twelve inches high and three inches wide. They were nine 

 inches apart ; and when a soft iron cylinder, three quarters of 

 an inch in diameter, and twelve inches long, was put across 

 from one to the other, it required a force of nearly one hun- 

 dred pounds to break the contact*. (Phil. Trans, of the Royal 

 Society of London, for the year 1832. Part i. page 135.) 



* There is a material difference in the proportions of magnitude and 

 power of this magnet, and of that which I described in the Phil. Mag. and 

 Annals lor March 1832. Here are 450 bars, which collectively weigh at 



