Unit resistance is that of a conductor through which unit electromotive force between its ends can send a unit current.

Unit capacity is that of a condenser which contains unit quantity when charged to unit difference of potential.

IV.  MAGNETIC UNITS.—­

Unit magnetic pole is that which repels an equal and similar pole at unit distance with unit force in air.

Strength of magnetic field at any point is measured by the force which would act on a unit magnetic pole placed at that point.

Unit intensity of field is that intensity of field which acts on a unit pole with unit force.

Moment of A magnet is the strength of either pole multiplied by the distance between the poles.

Intensity of magnetisation is the magnetic moment of a magnet divided by its volume.

Magnetic potential.—­The potential at a point due to a magnet is the work that must be done in removing a unit pole from that point to an infinite distance against the magnetic attraction, or in bringing up a unit pole from an infinite distance to that point against the magnetic repulsion.

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.