_Dielectric strength_ refers to the ability of an insulating material to resist rupture by high voltage, measured by the voltage necessary to effect a disruptive discharge through it. _Insulation resistance_, on the other hand, refers to the _ohmic_ resistance _offered_ by an insulating material to an impressed voltage, tending to induce a breakage of current through it. The term _dielectric_ is used as a synonym for _insulator_, in the sense that a charge on one part of a non-conductor is not communicated to any other part. A charge given to a conductor spreads to all parts of the body. A dielectric possesses the property of transmitting electric force by _in_duction but not by _con_duction. A charge on one part of a non-conductor or dielectric is not communicated to any other part. Jeans suggests that since the presence of magnetic energy is always associated with charges in motion, whereas electrostatic energy is present when all the charges are at rest relatively to each other, it may be proper to identify electrostatic energy with potential energy, and magnetic energy with kinetic energy[14]—i. e., energy due to motion of particles, rather than to energy of position, as of a coiled spring. Statical energy is distinguished from “current electricity” by the fact that it accumulates on various bodies—is stored up—and as soon as proper connections are made, it discharges instantly. Statical electricity is used by physicians in electrical treatment of diseases and in X-ray work. Machines have been constructed that will produce very strong charges of statical electricity. If a sufficiently large charge of electricity accumulates upon an insulated conductor in an electrical machine, it finally discharges itself, passing through the air to the nearest body. A flash of lightning is the result of an overcharge of statical electricity accumulated upon cloud particles, and may pass from cloud to cloud or descend to the earth.[15] Careful drivers of gasoline-tank wagons allow an iron or steel chain to drag on the roadway from a metallic connection, which conducts any surplus “static” to the ground. Failure to provide for such an emergency sometimes results in a terrific explosion with consequent loss of life. About the beginning of the nineteenth century, the Italian scientist, Alessandro Volta (1745-1827),—and other physicists—discovered what has been called, after Volta, _voltaic electricity_, a current generated by chemical action between metals and different liquids as arranged in a voltaic battery. The term “volt”—the electromotive force which performs work at the rate of one joule per second (one watt) in producing a current of one ampere—was similarly derived.