A Collection of Meters

Moving Coil Meter

 A nice little high quality bakelite bench meter, which measures about 4.5" square, was bought at a fleamarket in Antwerp in October 2000.

It was probably made in Nov 73 if the small inscriptions on the bottom of the dial are a clue. I'm not sure what it is exactly but it has a switch which selects DC or AC and it's clearly a precision piece of gear as it has two mirror scales each calibrated 0-100. The fsd of each is the same but the characteristics of the internal rectifier are slightly cramped at the low end of the AC scale making it non-linear. At the top are three sockets marked ~/+, - 1mA 0.1V, ~ 1mA 1V. Presumably the thing is designed for these specific ranges? Anyone any idea what it was made for?

Telsen, Multimeter

 This little Telsen meter probably dates from the late 20's or early 30's


GEC, Voltmeters

 ...and this one from later. It's possible that this one has a tale to tell as it only reads up to about 180 volts AC/DC. Before the mains supply was universally 240 volts AC each connurbation had its own local standard. The area around Bournemouth, where this meter probably spent it working life, had DC mains about half the present day voltage until WWII and in some places for some time after that.

Below, a second GEC Test Meter. This one is based on a 5mA movement and is calibrated for 4 ranges: 2v, 5v, 20v and 50v, selectable via the screw terminals, mounted on ebonite, at the top.

This one has the legend "USE IN A VERTICAL POSITION", clearly no lying down on the job.

It feels heavy so I weighed it. The two sets of kitchen scales ran out of steam at 5 pounds and 8 pounds so I had to find a proper weighing machine. At 12.5 pounds it is more than twice the weight of a standard AVO, which I always thought was fairly heavy but in fact is a mere 5.8 pounds.

The case and front panel appear to be made of cast iron, and like the example above it has a mirror scale for precise readings free from parallax errors

When was it made? Maybe someone knows? It looks like it may be 20s or early 30s but may be even older?


BPL, Multimeter

 This is an old "British Physical Laboratories" multimeter picked up at a car boot sale. These sort of things were OK but suffered from a major drawback. As they were powered by batteries and had to be switched ON they invariably got left ON and ended up with flat batteries. Note the paper label which told the user to switch OFF when not in use. I can't really understand the logic as if one was going to forgot to switch it OFF... one certainly wouldn't remember to read the label!

Radio Testmeters

 

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3

 

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5

 

6

 No.1

A pre-war German testmeter in new condition.

No.2

Testmeter, purchased for £1, is unusual because it's calibrated 0-6 volts and 0-15 amps. Perhaps it was for checking old wireless accumulators? It looks as if it might date from the 20s or early 30s.

No.3

This is calibrated 0-9 volts and 0-180 volts, probably dating from the introduction of battery valves with 6 volt heaters.

No.4

These type of little meters are only a couple of inches across and were made for checking radio batteries. Two scales are usually provided, selected by prodding the appropriate leg at the battery, with the flying lead being connected to the negative terminal. Some were made from brown bakelite and others had a metal case,some were "foreign made" and a few were British. Most read 0-6 volts and 0-120 volts.

No.5

Single scale, and devoid of a name.

No.6

This one has a logo incorporating the letters "SC" which represents the logo of "Schoeller & Co. elektrotechnische Fabrik GmbH, Frankfurt am Main" - a former German manufacturer of measurement instruments ( thanks to Gerrit Raschke for this identification).


AVO Test meters

 

AVO Model 40

 

AVO Model 8

 

AVO Model 7

 On the left, an old AVO from 1945 donated recently complete with its original leads.

This one has been stamped with a code for what might be an RAF Repair Depot.

These meters remained basically unchanged for 50 years. They could be dropped off the bench and still survive.

Electrically, they were almost indestructable, having a movement cut-out which operated when the needle swished across the scale and whacked the end stop too hard.

My Mk8 suffered a calamity recently. I can't remember when it happened but half a dozen resistors inside the case had burnt out. I fitted standard types, selecting their values on a digital meter.

The only disadvantage of these meters is the battery used for the 100k ohms scale which is a not very common miniature 22 volt type and quite expensive. Other ohms ranges make do with a D size 1.5 volt cell.

Centre, A grubby AVO Model 8

On the right, a nice clean AVO Model 7 that arrived in a perfect leather case

 I've repaired a few AVO meters. The symptoms were usually readings miles out and not working on some of the current ranges.

Inside the case can be seen lots of high stability resistors, such as 450ohms 0.3%. I measured several of these and found one to be very wide of the marked value and another open circuit.

As I didn't have a 450ohm and a 1.23kohm I made them up from metal oxide types.. 430 plus 22 ohm and 1.2kohm and 22 ohm.

This restored the accuracy and the missing current ranges.

 Switchable Milliameter

From the chalk it looks like this anonymous meter suffered a catastrophic problem?
 

 Fortunately not dead though. After a fall the needle had jumped out of its bottom bearing and the coil had jammed in the magnet gap.

An hour or so with steady hands had the meter back to a working state with all ranges seemingly accurate. Just a little white filler in the engravings for the ranges and it'll be as good as new.
 

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