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November 2018
A voltmeter in your pocket!

A voltmeter in your pocket!

01 November 2018

Keith Wilson - Electrical engineer

Do you own a voltmeter? If you work in the electrical or electronics sectors, your answer may well be ‘yes’, but if you’re simply a member of the general public, it will almost certainly be ‘no’. This wasn’t always the case, however. In the late 1920s and throughout the 1930s, a voltmeter was a very popular gadget to have in your home. But why?

The reason is directly related to the most popular form of home entertainment of the era: the radio or, as it would have been called back then, the wireless set. Television was still at the experimental stage in those days so if you wanted entertainment at home in the evening, you could either make your own or to listen to the radio. It’s not hard to work out which was the most popular choice. There was, however, a catch.

Unless you were prepared to settle for a crystal set, which at best would give quiet headphone reception of the local station, your radio needed electricity to power it. And most people couldn’t simply plug into a convenient wall socket because the majority of homes weren’t connected to a public electricity supply. The alternative was to use batteries, but as the transistor hadn’t yet been invented, we’re talking about valve (tube) radios and they need big, expensive batteries.

In fact, valve radios need at least two batteries: an HT battery (also called a B battery in the USA) to supply anything from 60 V up to 120 V, and an LT battery (or an A battery) to supply 2, 4 or 6 V for the valve filaments. In practice, dry batteries were usually used for the HT supply and rechargeable lead-acid accumulators for the LT supply. The HT batteries were very expensive (price fixing was legal back then, and almost universal) so you really wanted to make sure that, if your radio started to work less well than it had been, it definitely was the battery that needed replacing and not some other fault. And of course, if there was a programme you particularly wanted to hear, it was good to know that the battery – or even the accumulator – wouldn’t let you down in the middle of it.

The solution was to buy a voltmeter so you could keep an eye on those all-important battery voltages. If you could afford it, the instrument of choice was an AVO Minor made, of course, by one of the companies that ultimately formed part of the Megger organisation. But most folk needed something simpler and cheaper, so a vast range of pocket-watch voltmeters, like the one in the picture, appeared on the market.

For battery testing, sensitivity isn’t  an issue, so these little gadgets have a moving-iron movement and, in the one shown at least, the resistors have been wound rather crudely by hand. There’s no attempt at damping, so the pointer oscillates enthusiastically when making a reading! Two ranges are provided so that both HT and LT batteries can be checked.

Accuracy? Don’t even ask! But that wasn’t much of an issue either as the user would quickly come to know what reading meant a battery was good and what reading meant it was close to expiring, irrespective of the actual numbers indicated on the scale.

And what about safety? Well, a 120 V battery can deliver an unpleasant shock but it’s unlikely to prove fatal, so despite their almost non-existent insulation and metal case, these little voltmeters weren’t particularly hazardous when used as intended. Let us hope, however, that no one, full of the joy of having the public electric supply connected to their house at last, decided that it would be a good idea to check the supply voltage! These little voltmeters rapidly disappeared as more and more homes were provided with mains electricity but, despite their cheap design and even cheaper construction, in their time they satisfied a need and many a wireless owner must have been glad to have one in their pocket!