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Old 11th Feb 2018, 12:16 am   #18
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,736
Default Re: 'Record' Bond Tester (1944 and 1957)

Could be!

Anyway, having had chance to study the A.P. kindly scanned by Andy, it's pretty clear that both the Record Bond Tester and the Evershed & Vignoles Bonding Tester are effectively the same thing, with detail differences but meeting the same Air Ministry performance specification "E&I.417". Knowing that both instruments were originally supplied with two test leads, lead 'A' being 6 feet long having a twin-spike probe and lead 'B' being 60 feet long with a single spike probe, makes it possible to put the final pieces into the jigsaw.

On Richard's Radios website, there is a picture of the interior of the 'B' lead probe from an Evershed & Vignoles instrument, and it contains two small spirals of resistance wire. I've been in correspondence with Richard and I hope he won't mind my including this link. It seems that these resistances were included to bring the total resistance of the leads up to the value required to ensure the instrument read correctly. I have no doubt that the Record probes contained similar resistances.

I don't know the cross-sectional area of the original leads, but if we assume that they were the imperial equivalent of 0.5 sq mm (the smallest mains flex easily available today) which has a resistance of 39 ohms per kilometre, then the combined length of leads 'A' and 'B' of 66 feet (20metres) would have a resistance of 0.78 ohm. As I've discovered by experiment with short test leads of negligible resistance, the meter needs 1.89 ohms in series with its deflection coil to read true, so the probe resistance would be about 1.1 ohms. I guess this design gave the manufacturer the option of making available shorter or longer 'B' leads, up to about 160 feet in length, by simply altering the value of the probe resistors and not having to change the instrument internally at all.

Thanks again for everyone's interest.
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