UNIT CURRENT is that which in a wire of unit length, bent so as to form an arc of a circle of unit radius, would act upon a unit pole at the centre of the circle with unit force. UNIT QUANTITY of electricity is that which a unit current conveys in unit time. UNIT ELECTRO-MOTIVE FORCE or DIFFERENCE OF POTENTIAL is that which is produced in a conductor moving through a magnetic field at such a rate as to cut one unit line per second. UNIT RESISTANCE is that of a conductor in which unit current is produced by unit electro-motive force between its ends. UNIT CAPACITY is that of a condenser which will be at unit difference of potential when charged with unit quantity. Electric and magnetic force varies inversely as the square of the distance. PRACTICAL UNITS OF ELECTRICITY. RESISTANCE-R.--The Ohm is the resistance of a column of mercury 106.3 centimetres long, 1 square millimetre in cross-section, weighing 14.4521 grammes, and at a temperature of 0 degrees centigrade. Standards of wire are used for practical purposes. The ohm is equal to a thousand million, 10^9, electromagnetic or Centimetre-Gramme-Second ("C. G. S.") units of resistance. The megohm is one million ohms. The microhm is one millionth of an ohm. ELECTROMOTIVE FORCE--E.--The Volt is that electromotive force which maintains a current of one ampere in a conductor having a resistance of one ohm. The electromotive force of a Clark standard cell at a temperature of 15 degrees centigrade is 1.434 volts. The volt is equal to a hundred million, 10^8, C. G. S. units of electromotive force. CURRENT--C.--The Ampere is that current which will decompose 0.09324 milligramme of water (H2O) per second or deposit 1.118 milligrammes of silver per second. It is equal to one-tenth of a C. G. S. unit of current. The milliampere is one thousandth of an ampere. QUANTITY--Q.--The Coulomb is the quantity of electricity conveyed by an ampere in a second. It is equal to one-tenth of a C. G. S. unit of quantity. The micro-coulomb is one millionth of a coulomb. CAPACITY--K.--The farad is that capacity of a body, say a Leyden jar or condenser, which a coulomb of electricity will charge to the potential of a volt. It is equal to one thousand-millionth of a C. G. S. unit of capacity. The micro-farad is one millionth of a Farad. By Ohm's Law, Current = Electromotive Force/ Resistance, or C = E/R Ampere = Volt/Ohm Hence when we know any two of these quantities, we can find the third. For example, if we know the electromotive force or difference of potential in volts and the resistance in ohms of an electric circuit, we can easily find the current in amperes.