More test meters
Minor, Insulation Tester
This isn't very old...it's a
megger once used by the GPO with a built-in high voltage generator
used for testing insulation resistance. As you can see the scale
is marked up to 20Megohms.
A genuine Evershed &
Vignoles "Megger". This meter allows one to measure
very high resistances or test insulation leakage. The case contains
a DC generator which develops a high voltage when the handle
is turned. Most meters employ a small battery for this type of
measurement. An old AVO uses a 1.5 volt cell for normal resistance
and a 22 volt battery for high resistance measurements but even
the latter gives a cramped scale for really high resistance.
This Megger develops a nominal 500 volts and can therefore give
a good indication of really high values of resistance. The accuracy
depends on two things; firstly the efficiency of the internal
generator and secondly the speed at which one turns the handle.
At low rotations the output voltage is less than 100 volts but
at high rates the voltage can reach 800 volts or more. I haven't
opened up the thing to investigate a rattle when its shaken,
but certainly the results one can obtain with this example must
be very variable. The notice tells you to turn the handle at
120rpm for accurate results. I found that hard to do in practice
but no doubt an experienced user could manage. The calibration
reads 1.5 Mohm at half scale. With a 500 volt terminal voltage
that represents 333 microamps or 166 milliwatts and one can easily
distinguish between readings of 50 and 100 megohms. When was
it made? I'd guess about 1947 but I've really no idea.
small battery testmeter from 1944
Competitor to the AVO,
this is Sangamo Weston Model E772 "Analyzer". This
uses a relatively small switch coupled to lots of sockets for
different types of readings plus a separate switch for AC or
DC. You'll perhaps note that the scale carries an Air Ministry
logo and identification code so the multi-meter will probably
date to WW2. The scale has suffered from exposure to temperature
differential or strong light and needs sticking nback in place..
not an easy job.
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.
Dorman Smith IMPSC Meter
This is a specialist meter for
measuring what's termed the "Prospective Short-Circuit Current"
at either a 240 Volt or 415 Volt mains supply.
The meter can read from 200
to 20,000 amps.