We have now given some explanation of the manner in which electrical phenomena can be produced,—viz., by the Electric Machine and by the Leyden Jar,—but we must not expect to find any electricity inside any charged body. It has been proved that all the electricity is upon the surface of bodies, even if in varying quantity, and that equal quantities of electricity are always produced when bodies are excited by friction, but the _kinds_ are different. The rubbing body is of one kind, the body rubbed another, and consequently the forces neutralize each other. The two forces or kinds of electricity we have seen repel or attract each other, and we can imagine the farther they are apart the less will be the force, and the _rate of diminution of force_, according to distance, is ascertained by an ingenious apparatus called a “Torsion” Electromoter, which was constructed by Coulomb, and was frequently used by Faraday. Perhaps some people may not be aware of the term “torsion.” It means twisting, and “the torsion of a thread suspended vertically is the force tending to twist the lower extremity when the upper end is turned through an angle.” This instrument is really an Electromoter, and is not considered suited to beginners, and it is scarcely accurate in its workings. We need not therefore describe it in detail. There are some excellent Electromoters, the Elliott being, we believe, the best for use. A full and detailed description of the Quadrant Electromoter will be found in Mr. Gordon’s treatise on Electricity. _Recapitulation of foregoing Chapter._ So far, we have seen there is electricity in everything, although some bodies are termed conductors and others non-conductors; though, as in applying the terms heat and cold, we must remember that no body is entirely devoid of electricity, and no body is therefore an absolute _non_-conductor any more than any object is absolutely devoid of heat. Faraday, indeed, was of opinion that “conduction and insulation are only extreme degrees of one common condition”; they are identical both in principle and action, except that in _conduction_ an effect common to both is raised to the highest degree, and in the case of insulation it occurs in an almost insensible quantity.