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Experiencing the thrill of movement in analogue meters

29 Apr 2015  | David Ashton

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There are various other names for moving coil meters: Galvanometers (after Luigi Galvani, who was one of the first people to play around with electricity) and D'Arsonval movements or Weston movements, after the scientists who invented and developed this type of meter.

Pumping iron
Another common type of meter is the Moving Iron meter. Here, the coil is fixed, usually wound around a fixed piece of iron, and another a piece of iron is suspended in the magnetic field that is generated when current passes through the coil. The current magnetizes both pieces of iron, which repel each other, causing the moving one to move and deflect the indicator needle. This type of meter can be made very rugged, and they are consequently not as delicate as the moving coil types. They are a lot less sensitive, less linear (especially at low currents) and less accurate. Besides their ruggedness, another nice thing about this type of meter is that they are just as happy with AC as with DC. So they are well suited to things like battery chargers, mains voltage, current meters, and the like, where great accuracy is not needed and ruggedness may be of great advantage.

Moving Iron meter movement.

Moving Iron 5A meter face. Note the non-linear scale, particularly at the ends. Note also the AC/DC symbol underneath the "A" and the symbols to the left of the needle.

There are other types of meter—for example Taut Band movements use taut bands to suspend the coil rather than springs, giving less friction—but the above two are the types of meter that you are most likely to encounter. This is pretty universally the symbol used for a meter:

Apart from what type of meter you have, the most important thing you need to know about a meter is its FSD—the current needed to deflect the needle to the end of the scale. [Note to Max: I wonder why they don't call it Full Reticule Deflection? :-)] On a bare meter this will usually be given as a DC current, though on a meter which has already been adapted for a specific purpose it may be in another unit—Volts DC or AC, or even something like temperature. Generally the most sensitive meter you can get is 50µA FSD, and they are getting rarer, though D'Arsonval made one that could detect 10µA! As above, meters basically respond to current, but you can get them to read different parameters by using shunts, multipliers, rectifiers and the like. This is covered further in the next article in this series.

Going ballistic
Meter movements have what is called ballistics, which is how the meter responds to a step current applied to it. A big meter with a long needle will have a lot of inertia, the movement of the needle may encounter air resistance, and the inductance of the coil may have some effect, so it will not respond very quickly and may take some time to deflect to the correct reading. Smaller movements will rapidly move the needle to the desired point and may even overshoot and oscillate slightly before coming to rest. Meters may have vanes or even dash pots to damp the movement if this is undesirable, but there were ballistic meters where the deflection was proportional to the total charge applied to it. These were used for testing telephone lines. Analogue phones have a capacitor in them—usually around 2µF—and when a voltage is applied to the line you get a "kick" of current, and these meters were designed to maximise the kick to indicate the health or otherwise of the line, and a skilled operator could tell how many phones you had connected as well! Some meters intended for broadcast audio use have specified ballistics, so that momentary levels that would overmodulate a transmission can easily be seen, and there are standards for this too, notably from the British Broadcasting Corporation (BBC) but many other international standards organisations as well. Audio meters are usually calibrated in decibels (dB) or the closely related volume units (VU)—here is a typical example:

You also get Mirror Galvanometers—where the movement moves a small, light mirror rather than a long, heavy needle. In days of old this would deflect a light beam onto a scale, which could be large, giving the effect of a long needle without the weight or inertia. These days they are used to deflect laser beams for bar code scanners, micro marking, welding and cutting, effects in discos and the like. Scanlab is a company that specialises in mirror galvanometers and scan systems that use them, and their website is instructive if these interest you.

Be sure to check back for the next article in this series where I look at shunts and multipliers and introduce additional types of meters.

About the author
David Ashton contributed this article.

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