Техническое чтение для энергетиков. Бухарова Г.П. - 25 стр.

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27
future.
PRACTICAL UNITS
The three practical units, the ohm, ampere, and volt, provide standards for
comparison. They are defined as follows –
The ohm is the first primary unit, and the international ohm is defined as the
resistance offered to an unvarying electric current by a column of mercury at the
temperature of melting ice, 14.4521 gm. in mass, of uniform cross-sectional area and
of length 106.300 cm.
The ampere is the second primary unit. The international ampere is the unvarying
electric current which, when passed through a solution of nitrate of silver in water, in
accordance with a specification, deposits silver at the rate of 0.00111800 gm. per
second.
The volt is the third primary unit and is the electric pressure which, when applied
steadily to a conductor whose resistance is one international ohm, will produce a
current of one international ampere. Further, the international watt is the energy
expended per second by an unvarying electric current of one international ampere
under an electric pressure of one international volt.
In addition to the practical construction of the ohm as defined above it may be
derived in absolute measure as it has the dimensions of a velocity.
The original c.g.s. ampere was based on the magnetic effect of a current instead of the
present electro-chemical effect.
M.K.S. UNITS
An alternative absolute system is based on the dimensions metre, kilogramme, and
second. This is only one of many possible alternatives containing any multiple or
sub-multiple of the metre and the gramme. It has the advantage over the c.g.s. system
of simpler identity between the absolute and the practical units in that it removes the
powers of 10.
For the practical applications of the m.k.s. units a fourth unit is required in order to
define all the practical units. This has been taken to be the unit of resistance, leading
to the expression M.K.S. Ω units. But the present agreed fourth unit is the term µ
0
, the
permeability of free space, this being more fundamental. This applies of course to the
m.k.s. system of electromagnetic units, providing M.K.S. µ units. Electrostatic units
are not used with the m.k.s. system.
If µ
0
is taken to be unity in the c.g.s. system it becomes 10
-7
in the m.k.s. system (but
if rationalized it is 4π×10
-7
).
ELECTRIC CURRENT
Ever since Volta first produced a source of continuous current, men of science

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