UNIT DIFFERENCE OF MAGNETIC POTENTIAL.—Unit difference of magnetic potential exists between two points when it requires the expenditure of one erg of work to bring an (N. or S.) unit magnetic pole from one point to the other against the magnetic forces.
V. ELECTRO-MAGNETIC UNITS.—
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.