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possible, to limit the amount of current consumed by it, or else a
drop in potential due to the meter would occur and the pointer
indication would not represent the true potential difference across
the circuit.
Wattmeters. - The measurement of the power in a D.C.
circuit at any instant can be achieved by means of an ammeter and
voltmeter as the power in watts is the product of the current and
the voltage. With a.c. circuits however, the instantaneous values
are always changing. To measure A.C. power correctly, therefore,
it is necessary to use the third instrument to measure the phase
difference. The normal practice, however, is to combine these
three instruments in one which will give a direct reading of power
in watts.

9. Care of the Electrical Equipment

As a rule electrical equipment operates reliably. Still it does
not mean that it deserves no attention. It is necessary to give the
equipment frequent inspections, keep it well cleaned, lubricated
and repaired. Undue heating, vibration, sparking should be
immediately removed.
Heating may be due to overload or to a short circuit between
turns, lack of oil in bearings, vibration may be due to improper
foundation, unbalance in the moving parts of the machine.
Conductors may get heated because of overload or by reason
of damage of the insulation of the conductor.
An electrical machine of any kind requires certain conditions
under which it may operate reliably: temperature and freedom of
access of surrounding air, need for protection against dirt, dust,
type and duration of load, etc. Rotating machines should be placed
on solid foundations. Conductors should be protected against
mechanical damage. All measures or safety precaution must be
undertaken.

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