# What is the difference between a galvanometer, an ammeter and a voltmeter?

I'm looking for basic differences b/w the three with respect to nature of the circuit as well i.e either it is connected in parallel or series, and also the reason as to why they're connected the way they are? In what cases are these three used?

Galvanometer is a basic electromechanical instrument for measuring small currents. Old style analog ammeters and voltmeters are built around galvanometers.

It is an electromechanical instrument because it translates electrical current into a position of a mechanical pointer.

The heart of a typical galvanometer is a coil, which turns in the magnetic field of a permanent magnet, when a current flows through it. To measure the current in a circuit, the galvanometer coil has to be inserted in the circuit.

Galvanometers are typically made very sensitive, i.e., the deflection of a pointer per unit current is relatively high and, therefore, a full scale current is relatively low, commonly on the order of $$100\mu A$$, which limits the direct measurements to relatively small currents.

Ideally, the resistance of the galvanometer coil should be zero, in which case its insertion would not change the current in the circuit. In reality, the resistance is not zero, so, for accurate current measurements, the resistance of a galvanometer has to be small relative to the resistance of the rest of the circuit.

To overcome these limitations, a relatively small resistor (shunt) is connected in parallel with the coil, so that only a small fraction of the current in a circuit flows through the coil. Since the ratio of the coil resistance and shut resistance is known, the actual current in the circuit (a sum of the shunt current and the coil current) could be calculated. So, with a number of switchable shunt resistors, a galvanometer could be converted to an ammeter.

A galvanometer can also be used to measure voltage. If the coil of a galvanometer is connected between two points of a circuit, say, A and B, with some voltage between them, a fraction of the current in the circuit would branch off and flow through the coil. Since we know the resistance of the coil and can measure the current flowing through it, we can calculate the voltage between A and B.

That, however, would work well, only if the resistance of the circuit between A and B was much smaller than the resistance of the coil, so that only a small fraction of the current would branch off to the coil. Otherwise, the current in the circuit between A and B would decrease too much and the error would be significant. Also, if the voltage between A and B is high, the current flowing though the coil could exceed its limit.

To overcome these limitations, a relatively large resistor could be added in series with the coil. In this case, only a small fraction of the circuit current would be diverted to the coil. Knowing the ratio between the resistance of the series resistor and the resistance of the coil and measuring the current, we can calculate the voltage between A and B (a sum of voltages on the series resistor and the coil). So, with a number of switchable series resistors, a galvanometer could be converted to a voltmeter.

In summary, both ammeters and voltmeters, based on galvanometers, measure the current flowing through a coil of a galvanometer. To measure wide range of currents, the resistance of a galvanometer is reduced by adding shunt resistors in parallel with the coil. To measure wide range of voltages, the resistance of a galvanometer is increased by adding resistors in series with the coil.

• In the solid-state era there are other implementation of ammeters and voltmeters. That said you've done a very nice job of explaining the implementation details from the analog era. – dmckee --- ex-moderator kitten Sep 30 '18 at 16:44
• agree with @dmckee. Enjoyable reading. – niels nielsen Sep 30 '18 at 18:49
• Still not entirely clear what a voltmeter measures. According to Ashcroft and Mermin's Solid State Physics textbook, it's $V =-\int (\vec E+(1/e)\nabla \mu)\cdot d\vec l$. arxiv.org/abs/1502.05697 claims that it was demonstrated (back in 1997) that a voltmeter reads the electrochemical potential difference between two points, divided by the elementary charge. – thermomagnetic condensed boson Sep 30 '18 at 19:32
• @coniferous_smellerULPBG-W8ZgjR Now you are no longer making a marginal philosophical claim, but actually wrong. In a voltmeter built on a galvanometer there is neither significant chemical potential gradient nor significant thermal gradients over the coil. Context matters and the paper you and text your quoting are about very different contexts. – dmckee --- ex-moderator kitten Sep 30 '18 at 20:01
• @dmckee I am trying to understand it myself. So I stayed as general as possible. I do not think what I wrote is wrong. The fact that it may simplify to what you have in mind (when no thermal gradients) is a nice information, but I do not see why what I wrote is wrong. – thermomagnetic condensed boson Sep 30 '18 at 20:11

Well, V.F. has provided a comprehensive look at how a galvanometer work and how one builds ammeters and voltmeters on that basis, but I'd like to offer a high-altitude approach as well.

• An ammeter is any instrument for measuring current.
• A voltmeter is any instrument for measuring electric potential difference or EMF.
• A galvanometer is a particular type of analog ammeter using induction in a coil to deflect a needle against the resistance of a spring. They were often used as one component in building wider range ammeters and voltmeters.

If you go down to the hardware store, automotive store, or Radio Shack (if you can still find one) you may find a variety of cheap multimeters on sale. If you find one that has a mechanical needle it is probably implemented on a galvanometer, but most of them will have some kind of digital display and these are build on solid-state electronics; their working is quite different from the ones that V.F. discusses.

• According to arxiv.org/abs/1502.05697, it is claimed that according to "I. Riess. Solid State Ionics", a voltmeter reads the difference of electrochemical potential divided by the elementary charge, which differs from the electric potential difference. I'm pretty sure to have read something very similar in Aschcroft and Mermin's Solid State Physics textbook. – thermomagnetic condensed boson Sep 30 '18 at 19:11
• See p. 257 of Ashcroft and Mermin. $V =-\int (\vec E+(1/e)\nabla \mu)\cdot d\vec l$. – thermomagnetic condensed boson Sep 30 '18 at 19:18
• And so? Designing a working meter around the various limitation and non-linearities of solid state electronics is a demanding project, but it is a solved one across broad domains. Galvanometers have a fundamental elegance to them, but they come with their own limitations. For instance they can't measure variations that come faster than either the time constant of the inductor or the natural frequency of the of mass-on-spring system that you actually read off. In both cases you engineer around the limits of the technology. – dmckee --- ex-moderator kitten Sep 30 '18 at 19:19
• And so your claim "A voltmeter is any instrument for measuring electric potential difference or EMF." is not exactly correct (despite being popularized to a massive extent). – thermomagnetic condensed boson Sep 30 '18 at 19:34
• It is correct. Individual components inside the meter can not make that claim because they are affected by confounding variables, but the device as a whole can (with certain design limits). This is exactly the same problem that experimenters face in eliminating or correcting for systematic issues in experiment design and you can still get a correct answer with imperfect tools if you work hard enough. Or, if you insist otherwise, you have to count galvanometers out as well, because they fail to correctly indicate the value of a rapidly changing current. – dmckee --- ex-moderator kitten Sep 30 '18 at 19:43