[pmmunro]

Dave,

Thanks for taking the trouble to show us your fine work on the low-resistance Ohmmeter. As this is such a rare instrument, it's very fortunate that it came to you as you are able to appreciate its worth technically.

Once it's working to your satisfaction, will you be able to test its effectiveness? Low resistance is notoriously difficult to measure due to the influence of every contact point in the test circuit, not to mention within the test meter.

It would be interesting to know what its intended principal application was. At the time Megger, (not then part of the same company), hand cranked insulation and continuity testers were widely avaliable and would have been used to check low resistances.

Modern insulation/continuity testers seem to be quite good at measuring down to a few parts per hundred and readings which reflect this are regularly enetered on electrical test certificates.

The specification for a typical modern instrument (Megger MIT 310 series) is;

Characteristics - Continuity ranges

Measuring range: 0,01 Ω - 100 Ω
(0 -50 Ω on analogue scale)
Open circuit voltage: 5 V ±1 V
Short circuit current: 205 mA +10 mA -5 mA
Accuracy (at 20° C)
MIT300, 310, 320, 330: ±3% ±2 digits

Do you have the specification for the Low Resistance Ohmmeter?

The subject of low resistance measurement seems to have been of some interest to ACWEECO at the time as the attached advertisement shows. This show different approachs to low resistance measurement. The bonding meter is described as using the crossed coil technique and a 10Ah nickel iron accumulator - presumably similar in principle to a Megger and the "Milliohmmeter" used "An external 2V accumulator". The six terminals on top of the Milliohmmeter would suggest that it used the 4-wire technique.

PMM