_Conductors._--Sensible effects can be produced by electricity at great distances from the source, provided there be a medium of communication, that is, good _conductors_ to transfer it. When a glass rod is rubbed with a piece of silk, it becomes charged with an excess of, or positive, electricity, and at the same time the silk becomes charged with negative electricity. The glass rod will retain the positive electricity upon it for some time, unless touched with the wet hand, a wet cloth, a metal, &c., when it will instantly cease to be electrified. The electricity is then said to have been conducted away, and the bodies which allow it to run off the glass are called _conductors_ of electricity. Metals, water, the human body, charcoal, damp wood, and many other bodies are conductors. Those bodies which conduct electricity hardly at all, such as the air, silk, glass, sealing wax, gutta percha, india rubber, &c., are termed _nonconductors_ or _insulators_. Strictly speaking, all substances _conduct_ electricity in some degree, and a _nonconductor_ is merely a _bad_ conductor. In the following table the bodies are arranged in their order of conductivity, i.e. each substance conducts better than that which precedes it; the first-named body is the best insulator, and the last-named one is the best conductor. Dry air. Ebonite. Paraffin. Shellac. India rubber. Gutta percha. Resin. Sulphur. Sealing wax. Glass. Silk. Wool. Dry paper. Porcelain. Dry wood. Stone. Pure water. Rarefied air. Sea water. Saline solutions. Acids. Charcoal, or Coke. Mercury. Lead. Tin. Iron. Platinum. Zinc. Gold. Copper. Silver. Though two substances are near one another in the above list, they do not necessarily approach one another in their power of conducting. For instance, taking the conducting power of pure silver as represented by the number 100, then Pure Copper will be equal to 99·9, Gold will be equal to 78·0, while Zinc will be only equal to 29·0, and pure water, which is half-way down the list, will offer 6,754 millions more resistance than silver to the passage of the electric current. The metals being the best known conductors, are usually employed as the means of transferring the electric current from one place to another. _Electric Circuit._--The conditions attending this operation are different from those of any other known method of transmission. A complete _circuit_ must always be formed by the electric current, i.e. it cannot start from one place _A_, travel to another place _B_, and cease there, but the current must be completed before it can be said to have reached _B_. There cannot be a current of electricity without a means of recombination, which recombination must be at the _source_, or place of original disturbance.