This looks like it was
made in the late 40s or early 50s and uses a heavy diecast box
with a bakelite panel.
It's marked "Bridge Meg
Resistance Tester" and was made by Evershed & Vignoles.
It can test leakage of things
such as telephone lines by placing 1000 volts, derived from an
internal generator powered from the side handle, across the wires
or between a wire to ground and measuring the current in the
circuit. The meter, whose dial is under the metal flap, is calibrated
from zero ohms to 200 megohms, with the majority of the scale
registering over 1 Megohm.
It cost me £2 from a charity
shop in Boscombe and it's listed in my 1949 Instrument Manual.
Instruments like the Megger
is indispensible in the mining industry for checking insulation
of electrical wiring as the merest hint of a spark may produce
disastrous consequences if gas is present. The instrument provides
a maximum of 12mA output so as to limit the chance oif a lethal
shock (commonly defined as greater than 15mA) and presumably
to minimise the chance of causing ignition of any gas present
if a spark were to be produced.
The general form of the instrument
was invented by Mr Evershed in 1889 and it subsequently appeared
under the name "Megger" as long ago as 1904. It can
still be found 100 years later in much the same form. Accuracy
of the device depends on two principles.. the meter has to be
carefully designed and manufactured and the output of the internal
generator needs to be regulated. The latter is achieved by a
slipping clutch between the handle and the generator so that
the speed of the dynamo never exceeds 100 rpm, the design speed
for either 500 or 1000 volts, depending on the spec of the instrument.
This example incorporates a
Wheatstone bridge so that the resistance of a line may be measured.
Does this type of electrical
equipment perhaps carry the oldest name of any inventor as the
name Evershed, is to this day, still part of the manufacturers