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62
taining a standard pressure within a variation of 2 per cent.
The pressure in the houses may be different in different
cases, according to the needs or fancy of the consumer.
„ It is also a simple system to work, and the dynamos are
simple, and can be made in easily replaceable parts.
„ On the other hand the high pressure necessitates great care
and expense in insulation; the conveying of this high
pressure to, at any rate, one point on the consumer's
premises, and the risk of a contact between this circuit and
the low-pressure circuit inside the house, involves some risk
to life, although it may be reduced to a minimum by
proper regulations and precautions; the main cannot be
handled or connexions made when the pressure is on; the
regulation in the houses is not so good as in the mains, as
the Transformers act as a considerable added resistance,
and cause an increased variation of pressure in the power
as compared with that in the mains; the system cannot at
present be efficiently utilized for motive power, or for
flectric deposition or other chemical uses, such as charging
accumulators; and it is very doubtful whether a practically
successful alternating motor is likely to be brought out; the
system depends entirely on running machinery, and engines
and dynamos have to be kept in motion throughout the 24
hours; the general efficiency must be low when the supply
is near its minimum, a condition which, so long as it is
utilised for lighting alone, obtains during about 18 hours of
every day on the average, and a serious accident in the
generating station might stop the supply for a considerable
time. Alternating machines cannot be so readily connected
together to run in parallel circuit as continuous current
machines. The measurement of alternating current, pressure,
and power also offers considerable difficulty, and the
available instruments and methods are far more restricted
in number than is the case with the corresponding quantities
in continuous supply.
"The Chelsea system has the advantage of high pressure
between the generating and distributing stations, without
its being brought into the consumer's premises, and the
distributing stations may be so numerous that each one