The design of most of the
following mustn't have changed much for years when they were
made. Some look twenty years older than their indicated dates.
Apart from the first and last items, the following came from
Christchurch recycling centre and are marked "Hawker Siddley
Dynamics" and may have been dumped by British Aerospace
who now occupy the old Plessey site nearby. In the 1970s test
equipment began to look "high tech" instead of "quaint".
One has to have very small fingers to operate the latest gear
but the biggest advance is the use of transistors which do away
with long warm-up times. The advice, "switch on at least
an hour before measurements are taken to allow the equipment
to stabilise" is no more.
Heathkit home built
tester made from a kit of parts. It uses a "magic eye"
to indicate resonance
picture to see more details
Gavanometer or "Scalamp" made by Pye
This strange shaped instrument
uses a filament lamp and mirrors to provide extremely sensitive
current measurement. I think the scale reads +/- 7uAmp FSD. At
least it's calibrated in units to 7 either side of the centre
line and there are 10 subdivisions making it possible to measure
changes of 0.1uAmp. There's a multi-position switch allowing
the moving coil to be shunted to provide various sensitivities.
It is claimed that current changes down to 1 nanoamp can be detected
by similar instruments.
One use of this instrument is
to measure magnetic field, when it is known as a "Fluxmeter".
A coil is connected to the input terminals and this can be used
to measure changes in field strength. This technique pre-dates
measurement of magnetic field by nuclear magnetic resonance which
is a technique used in archaelogical searches.
Bridge by H.W.Sullivan
A design dating
back to pre-Victorian times this example was made in 1956 but
looks much older. It was used for making accurate measurements
of a component by comparing its parameters with known values.
Who invented Wheatstone's Bridge?
Not Mr.Wheatstone but a chap
called Christie or so an old Physics book in my library tells
me. None of my other books gives this information and I cannot
find another refenece to Mr.Christie. So who was he? Wheatstone
was the first to apply the bridge for practical measurements.
It's like saying a chap by the
name of Flintstone invented a useful looking round disk and a
Mr.Wheel first nailed them to the side of his cart.
Hands up anyone that knew that
Mr.Wheatstone invented the Telephone. Well he did, and in 1821,
no less than 55 years before it was patented by a Mr.Alexander
Graham Bell! Can anyone explain that conundrum!
Wire Potentiometer, Cambridge Instruments
equipment employs an external "Standard cell" which
can be from 1.017 to 1.019 volts and a 2V accumulator connected
across two pairs of its terminals. I haven't had time to study
it... perhaps someone would let me know what it might have been
used for? I guess it's a precision Wheatstone Bridge and it can
provide readings to 0.0002V which I think is 0.2millivolt or
Top, a basic model, Croydon
RBB5 supplying from 0.1 to 111,110ohms. Below, a Sullivan &
Griffiths Dual Dial equipment made in 1959 claiming zero reactance.
The latter can be achieved by using non-inductive resistance
elements or perhaps bifilar wound components. This model with
three knobs is interesting. One can switch in multiples of 100k,
10k, 1k, 100, 10, 1 : the last three by swinging the lower half
of the dial to the top. This saves panel space whilst keeping
the knobs and scales a handy size. It covers from 1 ohm to 1,110,000
Made by Taylor, this must
be one of their last valved equipments before transistors took
The simple analogue tuning dial
enabled receiver alignments to be made very easily, the engineer
being virtually oblivious to the sort of accuracy imposed by
later digital equipments.
Tuning drift was irrelevant
and in fact for most applications the accuracy provided by this
equipment was fine. The mathematics of superhet ganging alignment
does not require much in the way of pin-point accuracy when it
comes to setting up medium and long waves on an ordinary radio
Below, another similar styled
equipment from roughly the same date. This is a Grayshaw Instruments
Model SG50. I've used it recently and it provides a decent range
of output signals but alas, using modern analysing equipment,
one can see the output is not very clean, carrying lots of harmonics.
The dial is very simple but very clear and easy to read. Below
the picture is an advertisement for the SG50 in the December
1954 issue of Radio Constructor.