22 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XX.) 
others, including a vast number of opake and metallic bodies, and perhaps all 
except the magnetic metals and their compounds : and they also, through that con- 
dition, present us with the means of undertaking the correlation of magnetic pheno- 
mena, and perhaps the construction of a theory of general magnetic action founded 
on simple fundamental principles. 
2244. The whole matter is so new, and the phenomena so varied and general, that 
I must, with every desire to be brief, describe much which at last will be found to 
concentrate under simple principles of action. Still, in the present state of our 
knowledge, such is the only method by which I can make these principles and their 
results sufficiently manifest. 
i. Apparatus required. 
2245. The effects to be described require magnetic apparatus of great power, and 
under perfect command. Both these points are obtained by the use of electro- 
magnets, which can be raised to a degree of force far beyond that of natural or steel 
magnets ; and further, can be suddenly altogether deprived of power, or made ener- 
getic 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 already 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 extremities forming the poles are six inches from each other; 522 feet of copper 
wire 0' 1 7 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 magnetic 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 
