398 Dr. Faraday's Researches in Electricity. [Series xx. 



steel magnets; and further, can be suddenly altogether de- 

 prived ot" power, or made energetic to the highest degree, 

 without the slightest alteration of the arrangement, or of any 

 other circumstance belonging to an experiment. 



2246. One of the electro-magnets which I use is that al- 

 ready described under the term Woolwich helix (2192.). 

 The soft iron core belonging to it is twenty-eight inches in 

 length and 2*5 inches in diameter. When thrown into action 

 by ten pair of Grove's plates, either end will sustain one or two 

 half-hundred weights hanging to it. The magnet can be 

 placed either in the vertical or the horizontal position. The 

 iron core is a cylinder with flat ends, but I have had a cone 

 of iron made, two inches in diameter at the base and one inch 

 in height, and this placed at the end of the core, forms a 

 conical termination to it, when required. 



2247. Another magnet which I have had made has the 

 horse-shoe form. The bar of iron is forty-six inches in length 

 and 3'75 inches in diameter, and is so bent that the extremi- 

 ties forming the poles are six inches from each other; 522 

 feet of copper wire 17 of an inch in diameter and covered 

 with tape, are wound round the two straight parts of the bar, 

 forming two coils on these parts, each sixteen inches in length, 

 and composed of three layers of wire : the poles are, of course, 

 six inches apart, the ends are planed true, and against these 

 move two short bars of soft iron, 7 inches long and 2\ by 1 

 inch thick, which can be adjusted by screws, and held at any 

 distance less than six inches from each other. The ends of 

 these bars form the opposite poles of contrary name ; the mag- 

 netic field between them can be made of greater or smaller 

 extent, and the intensity of the lines of magnetic force be pro- 

 portionately varied. 



.2248. For the suspension of substances between and near 

 the poles of these magnets, I occasionally used a glass jar, 

 with a plate and sliding wire at the top. Six or eight lengths 

 of cocoon silk being equally stretched, were made into one 

 thread and attached, at the upper end, to the sliding rod, and 

 at the lower end to a stirrup of paper, in which anything to 

 be experimented on could be sustained. 



2249. Another very useful mode of suspension was to at- 

 tach one end of a fine thread, six feet long, to an adjustible 

 arm near the ceiling of the room, and terminating at the lower 

 end by a little ring of copper wire ; any substance to be sus- 

 pended could be held in a simple cradle of fine copper wire 

 having eight or ten inches of the wire prolonged upward ; 

 this, being bent into a hook at the superior extremity, gave 

 the means of attachment to the ring. The height of the sus- 



