Some Laboratory Testers

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.

Condenser checker

Heathkit home built tester made from a kit of parts. It uses a "magic eye" to indicate resonance

click the picture to see more details

Spot 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.

Wheatstone 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!

Slide Wire Potentiometer, Cambridge Instruments

This complicated-looking 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 200microvolt.

Resistance Boxes

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 ohms.

RF Signal Generator

Made by Taylor, this must be one of their last valved equipments before transistors took over.

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 set.

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