Donated by my old friend
Mike, this splendid piece of edwardian electrical equipment was
made by Everett Edgcumbe of London and carries the date of 4/3/1912.
It's designed for AC or DC mains having a potential of no more
than 200 volts and runs on a current, defined by a large wirewound
resistance, of 1.25Amps at the rated maximum voltage.It must
have been in use for many years as two of its terminals have
been replaced by modern types. The method of connection is shown on the inside
of its hinged lid.It's about 7 inches square.
Measurement of watts is not
an easy business and many types of wattmeter are around. I saw
one the other day designed in the 20s by a firm that made clocks.
It used a pendulum arrangement, governed by current passing through
the instrument, to indicate watts.
The one illustrated however
uses the product of volts and amps to move a meter across the
scale. The interior has a large toroidal coil which sets up a
field, against which a moving coil works to produce mechanical
movement. The mechanism uses a damping arrangement to steady
the pointer and in order to get consistent results the box must
be resting on a flat horizontal surface.
A large flat wirewound resistance
governs the current fed to the moving coil. The value of the
resistance is such that at 200 volts the current is defined as
1.25 amps. At these maximum values the meter reads 250 watts.
To use the meter at the modern mains voltage of 240 means that
the field coil and the ballast resistance will be over-run somewhat
and may get warmer than the designers intended.