Resistance Testing

You don't know just how lucky you are with a digital multi-meter!

Wheatstone Bridge

 The bridge is contained in a mahogany box with heavy brass hinges and marked on an exterior brass label

"W & J GEORGE LTD GREAT CHARLES STREET, BIRMINGHAM 3

F.E.BECKER & CO. HATTON WALL LONDON EC1." and has "NIVOC" in a diamond.

Inside on the ebonite panel are inscribed the same names together with "No.11180"

Using a sensitive centre zero meter and a battery the box could be used to measure the resistance of a telephone line for example. The internal resistances vary from 1 to 5000 ohms.

It appears to date from Edwardian times or before 1920.

Resistance Box

 This is a mahogany based unit carrying an ebonite panel with keys permitting a resistance up to a maximum of 110 ohms in steps of 1 ohm.

The instrument was made on 12/12/1922 as it is inscribed as such inside the wooden case, which also carries the initial "J". The underside of the ebonoite carries the score marks for drilling. There's no maker's name on the box so may have come from a cottage industry rather than one of the big manufacturers of scientific instruments.

Bakelite-cased resistors

 I have quite a number of these items, two of which are shown here..

These were made by Griffin & George Ltd, probably for a commercial laboratory or a maybe a school physics lab. Mine did not get as far as th ebench and are brand new and still in their original wrappings. They are roughly the size of headphone earpieces

Bridge Megger

 This looks like it was made in the late 40s or early 50s and uses a heavy diecast box with a bakelite panel.

It's marked "Bridge Meg Resistance Tester" and was made by Evershed & Vignoles.

It can test leakage of things such as telephone lines by placing 1000 volts, derived from an internal generator powered from the side handle, across the wires or between a wire to ground and measuring the current in the circuit. The meter, whose dial is under the metal flap, is calibrated from zero ohms to 200 megohms, with the majority of the scale registering over 1 Megohm.

It cost me £2 from a charity shop in Boscombe and it's listed in my 1949 Instrument Manual.

Instruments like the Megger is indispensible in the mining industry for checking insulation of electrical wiring as the merest hint of a spark may produce disastrous consequences if gas is present. The instrument provides a maximum of 12mA output so as to limit the chance oif a lethal shock (commonly defined as greater than 15mA) and presumably to minimise the chance of causing ignition of any gas present if a spark were to be produced.

The general form of the instrument was invented by Mr Evershed in 1889 and it subsequently appeared under the name "Megger" as long ago as 1904. It can still be found 100 years later in much the same form. Accuracy of the device depends on two principles.. the meter has to be carefully designed and manufactured and the output of the internal generator needs to be regulated. The latter is achieved by a slipping clutch between the handle and the generator so that the speed of the dynamo never exceeds 100 rpm, the design speed for either 500 or 1000 volts, depending on the spec of the instrument.

This example incorporates a Wheatstone bridge so that the resistance of a line may be measured.

Does this type of electrical equipment perhaps carry the oldest name of any inventor as the name Evershed, is to this day, still part of the manufacturers name?

Resistance Boxes

 A set of modular resistance boxes ranging from tenths of an ohm through to tens of thousands. These would permit the synthesis of a resistor from zero to a maximum of 100,000 + 10,000 + 1,000 + 100 + 10 + 1 or 111,111ohms.

Used in a bridge circuit it would permit measurements of 6 significant figures. To attain these levels of accuracy the equipment would need to be very accurately calibrated and not subjected to stress greater than it was designed for. To this end each of the boxes is marked with the maximum current allowed.

Gambrell Decade Resistance Box


Reference Battery 

This rather strange set-up is battery made up from laboratory standard reference cells. These things are not designed to supply power, in fact in the days before high resistance digital testmeters it was strictly forbidden to attempt to measure their terminal voltage.

These examples are all marked 5154B, 1.01859 volts @ 20 deg C and made by Tinsley & Co
 

 

 

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