More test meters

Record Minor, Insulation Tester

This isn't very old...it's a megger once used by the GPO with a built-in high voltage generator used for testing insulation resistance. As you can see the scale is marked up to 20Megohms.

Megger, Insulation Tester

A genuine Evershed & Vignoles "Megger". This meter allows one to measure very high resistances or test insulation leakage. The case contains a DC generator which develops a high voltage when the handle is turned. Most meters employ a small battery for this type of measurement. An old AVO uses a 1.5 volt cell for normal resistance and a 22 volt battery for high resistance measurements but even the latter gives a cramped scale for really high resistance. This Megger develops a nominal 500 volts and can therefore give a good indication of really high values of resistance. The accuracy depends on two things; firstly the efficiency of the internal generator and secondly the speed at which one turns the handle. At low rotations the output voltage is less than 100 volts but at high rates the voltage can reach 800 volts or more. I haven't opened up the thing to investigate a rattle when its shaken, but certainly the results one can obtain with this example must be very variable. The notice tells you to turn the handle at 120rpm for accurate results. I found that hard to do in practice but no doubt an experienced user could manage. The calibration reads 1.5 Mohm at half scale. With a 500 volt terminal voltage that represents 333 microamps or 166 milliwatts and one can easily distinguish between readings of 50 and 100 megohms. When was it made? I'd guess about 1947 but I've really no idea.

A small battery testmeter from 1944

Sangamo Weston Multimeter

Competitor to the AVO, this is Sangamo Weston Model E772 "Analyzer"

Wattmeter

Donated by my old friend Mike, this splendid piece of edwardian electrical equipment was made by Everett Edgcumbe of London and carries the date of 4/3/1912. It's designed for AC or DC mains having a potential of no more than 200 volts and runs on a current, defined by a large wirewound resistance, of 1.25Amps at the rated maximum voltage.It must have been in use for many years as two of its terminals have been replaced by modern types. The method of connection is shown on the inside of its hinged lid.It's about 7 inches square.

Measurement of watts is not an easy business and many types of wattmeter are around. I saw one the other day designed in the 20s by a firm that made clocks. It used a pendulum arrangement, governed by current passing through the instrument, to indicate watts.

The one illustrated however uses the product of volts and amps to move a meter across the scale. The interior has a large toroidal coil which sets up a field, against which a moving coil works to produce mechanical movement. The mechanism uses a damping arrangement to steady the pointer and in order to get consistent results the box must be resting on a flat horizontal surface.

A large flat wirewound resistance governs the current fed to the moving coil. The value of the resistance is such that at 200 volts the current is defined as 1.25 amps. At these maximum values the meter reads 250 watts. To use the meter at the modern mains voltage of 240 means that the field coil and the ballast resistance will be over-run somewhat and may get warmer than the designers intended.

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