The Current Elements.—In the action of the pair, three elements are to be considered —the eiectro-motive force, the resistance, and the strength of the current. The electro motive force is proportional to the force, tending to chemical action if we adopt the chemical theory, or, on the contact theory, to difference of potential produced by the contact of the two metals. It is measured directly by the charge,. or, as it is called, the potential (tension) which a cell gives to a delicate electrometer. In Thomson's reflect ing quadrant electrometer, for instance, a single Daniell's cell deflects the needle so much, that the spot of light moves some 24. in. from the zero-point of the scale. The relative electro-motive powers of the various forms of cells can be ascertained by the amount of deflection indicated by such an instruinent. The resistance in the circuit which is offered by the liquid of the cell and the iuterpolar wire or other connection, is that which tends, to reduce the current or flow of electricity produced by the electro motive force: As stated in Ohm's law, the current strength is equal to the electrodno five force divided by the resistance. The electro-motive forces of the different cells
can also be compared by observing the effect, on the current strength, of a given resist ance interposed in the relative circuits, whose own proper resistance has been previously known.
(Tails of the Carrent Elements. —The unit of electro-motive force now adopted by British electricians is called a vat. This is about 7 per cent less than that of a Daniell's cell. The unit of resistance is called an ohm or B.A. unit. 485 meters of pure copper wire 1 Millimeter in diameter offer an ohm of resistance; so does about of a mile of ordinary telegraphic wire (No. 8). The unit of current strength is called a farad, and is the amount of flow of electricity that would be produced in a second if a cell of a volt in power were to act in a circuit of an ohm of resistance. One million volts are called a megavolt; one-riillionth of a volt, a microvolt. The same proportion holds for a megohm and a microhin, a megafarad and a microfarad. In the centimeter, gram, second series of electro-magnetic units, an ohm is expressed as a farad as 10 and a volt as