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In The Art of Electronics 3rd, it mentions:

The traditional (and now largely obsolete) VOM (volt-ohmmilliammeter) multimeter uses a meter movement that measures current (typically 50μA full scale). (See a less-design-oriented electronics book for pretty pictures of the innards of meter movements; for our purposes, it suffices to say that it uses coils and magnets.) To measure voltage, the VOM puts a resistor in series with the basic movement. For instance, one kind of VOM will generate a 1V (full-scale) range by putting a 20k resistor in series with the standard 50μA movement; higher voltage ranges use correspondingly larger resistors. Such a VOM is specified as 20,000Ω/V, meaning that it looks like a resistor whose value is 20k multiplied by the full-scale voltage of the particular range selected. Full scale on any voltage range is 1/20,000 amps, or 50μA. It should be clear that one of these voltmeters disturbs a circuit less on a higher range, since it looks like a higher resistance (think of the voltmeter as the lower leg of a voltage divider, with the Th´evenin resistance of the circuit you are measuring as the upper resistor). Ideally, a voltmeter should have infinite input resistance.

From https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/what-is-a-meter/ , I know what does 'meter movement' refer to:

description of meter movement

But I don't know what does 'basic movement' refer to.

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    \$\begingroup\$ Basic : Torque from current causes deflection. A series R calibrates the current for FSD. \$\endgroup\$ Commented Apr 3 at 20:54
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    \$\begingroup\$ @TonyStewartEEsince1975 Thank you for your comment. \$\endgroup\$ Commented Apr 3 at 22:29
  • \$\begingroup\$ Are they using the term 'movement' in two different senses? In one sense meaning the physical structure of the device analogous to a watch/clock movement and in the second the visible movement shown by the meter hand against the dial markings? \$\endgroup\$ Commented Apr 6 at 9:24
  • \$\begingroup\$ @civitas About "Are they using the term 'movement' in two different senses?", the answer is NO. 'Movement' does not refer to the physical structure of the device analogous to a watch or clock movement. 'Movement' refer to a mechanical device(part) that is powered or controlled by electricity. Please see the answer written by user: Neil_UK that point out: So the 'movement', 'basic movement', and 'meter movement' all mean the electromechanical 'current in to needle deflection out' mechanism that sits at the heart of a 'meter'. \$\endgroup\$ Commented Apr 7 at 2:08

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Those are not well defined terms, and these articles are describing rather than defining them. So from context, it appears that ...

The 'basic movement' gets a resistor put in series with it to measure higher voltages. Therefore, the basic movement is just the moving coil and all the mechanics around it.

The 'meter movement' seems to be exactly the same thing as a basic movement, as a VOM uses one. For a VOM to use some sort of movement, it will need to switch different resistors in series with the coil, to make it able to measure different voltages.

So the 'movement', 'basic movement', and 'meter movement' all mean the electromechanical 'current in to needle deflection out' mechanism that sits at the heart of a 'meter'. To complete the meter, you add a case, a scale, and scaling resistor(s) to make the completed meter read what you want.

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In a good old-fashioned analogue meter it’s the coil. This is essentially a fairly sensitive ammeter that can serve as a voltmeter if it’s in series with a resistor or as an ammeter if in parallel with a shunt resistor. They can be used in other ways with more complex circuitry.

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I don't know what does 'basic movement' refer to.

The generic term of "galvanometer" applies to an indicator showing amount of current flowing through the meter.

The term "movement" refers to the delicate mechanical suspension that allows the indicator to rest at zero with no current, yet swing to a new position when DC current flows. A desirable goal is to swing proportional to current...in a linear way. It is also desirable for the indicator to settle to a new position quickly, with no overshoot. Gravity should not affect the zero position, nor affect linearity of the indicator.

Mechanical suspension of the coil:

  • A jewel bearing (perhaps sapphire or ruby) pivot point combined with weak return spring.
  • A taut-band suspension made of phosphor bronze or beryllium copper. May feed current to the rotating coil, and provides restoring torque to zero. Robust.

The term "basic" refers to the raw coil through which current flows - ideally as sensitive as possible. A typical multimeter might have a basic sensitivity where a full-scale indication corresponds to 50 microamps (perhaps 30 microamps) DC. The very fine wire of the coil might be temperature-compensated...often having end-to-end resistance of a few kilohms. A multimeter's most sensitive current scale may give you a clue to the basic movement's sensitivity.

External shunt resistors are switched in parallel with the basic movement to indicate larger currents at full-scale. Or external series resistors are switched to the basic movement to indicate large voltages at full-scale.
A multimeter adds this external switch and external resistors to expand the range of current or voltage that a basic movement can display.

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  • \$\begingroup\$ Thank you for your answer. \$\endgroup\$ Commented Apr 3 at 23:25
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A little history

In the 19th century, they had electrostatic voltmeters with very high (almost infinite) input resistance, but they were non-linear. At the same time, they had magnetoelectric ammeters ("movements") with very low (almost zero) resistance, which were linear.

Basic idea

This led them to the idea of creating a voltmeter by using an ammeter. To achieve this, they connected a voltage-to-current converter (a resistor) to the input of this current-to-deflection converter (a movement).

Building a voltmeter with an ammeter

Let’s trace the evolution of this idea from then until today.

Perfect ammeter

The ideal ammeter ("current-to-deflection converter" or "basic movement") has zero resistance; therefore, no voltage Vam is dropped across it and the current does not decrease.

schematic

simulate this circuit – Schematic created using CircuitLab

Perfect assembled voltmeter

To measure voltage, we connect a resistor R in series as a voltage-to-current converter.

schematic

simulate this circuit

Imperfect voltmeter

However, real ammeters have some resistance, e.g., 1 kΩ. Then, a harmful voltage drop Vam occurs across it, which is subtracted from the input voltage, and the current decreases.

schematic

simulate this circuit

Corrected voltmeter

Of course, we can correct the resistor's resistance by subtracting the ammeter's resistance from it.

schematic

simulate this circuit

Improved voltmeter

... but a more sophisticated idea is to compensate for this drop by adding an equal voltage. To achieve this, we can insert a voltage source in series and adjust its voltage Vam to be equal to the unwanted drop.

schematic

simulate this circuit

Op-amp implementation

In the practical circuit, an op-amp performs the role of the compensating source Vam. It adjusts its output voltage to be equal to the voltage drop Vam. As a result, the voltage-to-current converter (R) "sees" zero resistance (virtual ground).

schematic

simulate this circuit

Negative-resistance viewpoint

If we look at this circuit from a different perspective, we can imagine the op-amp output as a negative resistance with the same value as that of the ammeter. In this way, the negative resistance neutralizes the positive one and the result is 0 Ω (0 V).

schematic

simulate this circuit

Related links

Here are a few of my circuit stories dedicated to this solution:

Passive voltage-to-current converter (archived Flash movie)

How do we build an op-amp ammeter? (archived), short version

Op-amp circuit builder (archived Flash movie; click the ammeter symbol in the bottom right corner of the library)

How do we improve the real ammeter? (RG question)

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    \$\begingroup\$ Thank you for your answer. \$\endgroup\$ Commented Apr 5 at 1:36
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Lower image shows a D'Arsonal moving coil meter movement. The majority of analog meters use this meter movement.

It consists of a coil of wire made from an alloy with a low temperature coefficient, like manganin or constantin to make the meter temperature insensitive. The coil of wire is wound around, but not attached to an iron core.

Adjacent to the iron core is a pair of magnets connected by a C shaped iron segment as a path for the magnetic field.

Cited article uses 50μA. This is a very sensitive meter movement. Usually D'Arsonal meter movements have larger currents, like 1mA. This is the current that will cause full-scale deflection (FSD) of the meter movement.

The meter movement uses the same principle of electric motors. Two magnetic fields interacting. When a current is passed through the coil, the coil becomes magnetized. This magnet interacts with the magnetic field of the permanent magnets, causing rotation of the coil. Now attach a needle to the coil and we have an analog meter.

The stronger the current, the more deflection. Deflection is proportional to current in coil. 50μA = FSD. 25μA = half-scale deflection. 60μA = 20% past FSD, which would cause needle to hit mechanical constraints and possibly bend needle.

That is a meter movement.

From D'Arsonaval Meter Movement

Meyer Movement

Different analog meters use temperature insensitive resistors in series or parallel or series/parallel and diodes for ac. Voltmeter uses a series resistor to extend range of meter. Ammeter uses a shunt. Ohmmeter can use a battery with series resistors or series and parallel resistors. Resistors for ammeter can be small pieces of wire.

The example of a 1V (full-scale). The design is simple. 1V = FSD = 50μA. Ohm's law requires a total resistance of 20k. The meter movement is made of high resistance wire, say 5k, so we subtract that from the total.

Place a 15k resistor in series with the meter movement and we convert it from a device that measures current into a device that measures voltage.

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  • \$\begingroup\$ Thank you for your answer. \$\endgroup\$ Commented Apr 3 at 9:51
  • \$\begingroup\$ Never heard of using low TC wire instead of copper before. It would make sense for a meter dedicated to only voltage or current measurement, low TC required and high operating current OK for a current meter, high operating voltage OK for a voltmeter, but not for a switchable AVO type meter? \$\endgroup\$ Commented Apr 3 at 10:49
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    \$\begingroup\$ A 50μA movement was very common on analogue multimeters. It gave the meter a usable resistance of 20kΩ per volt DC. A higher current movement would load the circuit more. \$\endgroup\$ Commented Apr 3 at 14:01
  • \$\begingroup\$ Great for high end voltmeters, but wasted on ammeters. Out of 10+ meters I've had apart, only one had 5k/V. An excellent voltmeter. Rest were in the 1mA and 0.5mA range. Voltmeters with resistance close to measured resistor corrupts reading beyond stated accuracy. \$\endgroup\$ Commented Apr 3 at 16:31
  • \$\begingroup\$ @Neil_UK Dedicated multirange (1mA to 1A) ac and dc ammeters. Hard to get a shunt resistance accurate enough at low resistances. Few turns of different diameter high resistance wire. \$\endgroup\$ Commented Apr 3 at 17:32

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