selectable 115VAC or 240VAC, 50-60Hz
- Range: 0.001
to 1100 volts DC
lineup (none -
Marconi Instruments make a good deal of very
high grade test equipment for the defense industry. As this equipment
is replaced after about 20 years it is disposed of, usually cheaply at
This is how I came into posession of
unit. It dates from the mid-1960's (I think) and is ex-Ministry Of
Defence (see the arrow sticker lower right corner dated June 1970). It
is a transistorised unit driven by a selectable 240V or 115V mains
supply and has a working range of 0.001V to 1100V DC. It has three
sensitivity ranges of 2, 3 or 4 decimal places - the decimal point
being indicated by a neon lamp according to the 'Max Input' control
units are the predecessors of the now ubiquitous DMM - multi-meters
with digital readouts accurate to 3 or 4 digits - and are, in many
ways, still vastly superior.
what is a 'differential voltmeter' and how does it differ from a
'regular' voltmeter? It is really a precise voltage generator
coupled to a very sensitive ammeter and is used to measure DC voltages.
It is connected to the circuit with the + of the generator connected to
the + of the circuit (and - to -). The voltage of the generator is
increased until zero current flows. The voltage is then read off the
knobs of the generator. It's big advantage is that it has a virtually
infinite input impedance.
Guttery gives a more detailed explanation from the archives of the
Usenet group rec.antiques.radio+phono
"Most voltmeters work by 'sampling' the
voltage to be measured. The sampling cirucit is usually some kind of
voltage divider - with either a meter, or an amplifier driving a meter.
Depending on the circuit, the 'load' placed on the source of the
voltage to be measured can vary greatly - and is usually expressed in
ohms per volt.
Common multimeters range from cheap
ones, at 1000 ohms per volt, to pretty good ones at 20,000 ohms per
volt. Better meters (such as older vacuum-tube meters or the newer
field-effect transistor types) may have be as high as 10Megohm per
volt. But they all present some "load" to the source. One point further
- the "ohms per volt refers to the "full scale voltage" - so a good
multi-meter set to the 250V range will present a (250*20,000=) 5Megohms
load. The same meter on 0.5 Volt range will present a 10,000 load. In
most circuits -- this isn't a real problem. In a grid circuit sourced
off a 200K bias resistor - this load would cause the voltage reading to
be wrong - very wrong.
Enter the differential voltmeter. The
differential works differently. Instead of measuring the voltage
directly, it uses a form of balanced bridge so that it measures the
difference (differential - get it?) between a known voltage and the
voltage you are measuring. If there is no difference, then there is no
current drawn from the source (i.e. infinite impedence). So all of
those knobs you see form a Kelvin-Varley divider (voltage source) which
you set until the meter reads null - then the input voltage is equal to
(and read from) the dials. Obviously, you could use this meter for any
voltage measuring purpose, but mostly where you need to ensure that the
meter is not loading the source circuit in any way."
updated 2nd June 2008