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What happens if you connect a galvanometer to an AC source?

I have seen many answers online along the lines of there not being any deflections as the net average current/net average magnetic field is zero (depends on the site but I won't cite them as I don't wholeheartedly trust them from bottom of my heart.)

I argue that since there is indeed an instantaneous current we must have an instantaneous deflection, right? I also want to argue two more things on what we may observe if we were to do a live demonstration.

Firstly the galvanometer coil must get damaged after a while due to the eddy current (this is a moving coil galvanometer I am considering currently. I also heard there's a metallic core in the galvanometer to make the pointer come to rest immediately.) So I argue that the MCG must get broken after a while.

Secondly, assuming the coil doesn't break, we can easily imagine the maximum deflection being produced when there is maximum current flowing. That's what I am assuming in my mind. Since my AC current flows at 220 V, 50 Hz I assume there must be at least 50 times the pointer going from one end to the other in a single second which would be impossible to be seen by a human eye thus rendering the whole act of finding deflection pointless. Maybe the galvanometer won't even move due to the inertia. Would I need to have a spare moving coil galvanometer and an AC source to try that out?

JRE
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Adil Mohammed
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    "since there is indeed an instantaneous current we must have" an instantaneous _force_. How much "deflection" we get from that force in the few milliseconds before the force is turned around in the opposite direction depends on a number of different things (explained in answers, below.) – Solomon Slow Nov 16 '21 at 20:39
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    Some galvanometers (usually mirror type) can easily respond to 50Hz. – Spehro Pefhany Nov 16 '21 at 21:14

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It depends on several factors such as the frequency of the AC voltage and the mass of the galvanometer. A galvanometer is a passive DC ammeter that relies on the magnetic field induced in a coil surrounding a magnet to generate a torque proportional to the current in the coil. That, of course, moves the pointer to someplace on the calibrated scale where you read the current.

So an AC signal (without any DC component) should show a net of 0A. But the pointer may wiggle slightly or a lot depending on the factors mentioned above.

But as long as the current doesn't exceed the rated current of the meter and you don't do anything silly like generate a full-scale reversal and slam the pointer into the stop, it should be undamaged.

I've also see some galvanometers that have both a + and a - range with the 0 point in the middle.

You'll want to consult the specs for the galvanometer to be sure you are using it safely.

jwh20
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  • Thanks for your reply! " A galvanometer is a passive DC ammeter that relies on the magnetic field induced in a coil surrounding a magnet to generate a torque proportional to the current in the coil" you also mention 0A. Is this because there isn't enough time for a magnetic field to be established in our scenario? – Adil Mohammed Nov 16 '21 at 18:16
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    Due to the inductance of the coil and the mass of the coil and pointer the mechanism will not respond instantly. Since the average current over time for an AC signal is 0 Amps, the needle will not move or will move only slightly if the conditions are right. If you apply a slow enough AC signal the pointer will track it. Try it with a signal generator and a 1Hz signal for example. – jwh20 Nov 16 '21 at 18:18
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A galvanometer, or its descendant the D’Arsonval/Weston movement, would be destroyed with the direct application of 230V AC line as they’re current-based devices with a fairly low impedance.

Example: a typical high-sensitivity (50kohm/V) analog meter movement is 50uA full scale and 1000 ohms. A mere 50mV will deflect it full scale. 230V across this meter movement will vibrate it mercilessly until it destroys the coil with over 100W peak power. Basically, you'd see a brief buzz followed by a poof of smoke from the coil.

With that out of the way, what happens to a galvanometer if you apply (voltage- and current-limited) AC? The needle waggles back and forth. If the frequency is high enough, the needle will appear to not move at all, due to its mechanical limits and the coil inductance.

hacktastical
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  • If the frequency is high enough, the needle won't just APPEAR to not move at all. It will actually not move at all. – Laurence Nov 17 '21 at 13:19
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    @LaurencePayne [citation needed]. Are there quantization effects at play? – user253751 Nov 17 '21 at 15:39
  • @user253751 most of the answers are saying with a standard galvanometer it happens only at low frequency. I assume inertia/mass is the culprit – Adil Mohammed Nov 18 '21 at 13:39
  • Just inertia is enough! The point is that if it APPEARS to not move, this isn't some kind of illusion. It really isn't moving! – Laurence Nov 18 '21 at 14:09
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    Pedantry alert! As the frequency increases the needle motion decreases. Nonetheless it still moves, even if you visually can’t see it directly. A mirror galvanometer would show more, as a dithered light. There will be of course some frequency at which even that motion will be too small to detect, even indirectly. Recall that OP said 50Hz, which is easily seen as a moving needle by the naked eye, albeit blurred. – hacktastical Nov 18 '21 at 15:49
  • @LaurencePayne Inertia will reduce the amplitude arbitrarily low, but greater than zero, as the frequency gets arbitrarily high. – user253751 Nov 18 '21 at 18:56
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Or or maybe the Galvanometer won't even move due to the Inertia.

At 50 Hz, that will certainly be the case. If the frequency is low enough, the needle would follow the change in current. At some frequency, the needle will move, but it or some part of the mechanism will break due to the stress of repeatedly changing direction. A few days ago, I observed one cycle at a time a few times without apparent damage. I dropped a magnet through a coil attached to a galvanometer.

Here are a few frames of a movie of a BLDC motor, turned by hand, generating alternating current through a galvanometer.

enter image description here

  • That is pretty cool, could you share a picture of your galvanometer if possible? I assume you removed the iron core – Adil Mohammed Nov 17 '21 at 12:48
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    I removed the iron core and replaced it with a 0.6 m clear plastic tube. I connected 2 LEDs (one each direction) across the coil. It makes a nice demonstration at the local science museum, but is not robust enough to last very long. The galvanometer is not in the exhibit. Pictures later. –  Nov 17 '21 at 13:16
  • Added a picture. –  Nov 17 '21 at 19:02
  • Well I guess this is the most appropriate answer given that you did the thing. – Adil Mohammed Nov 18 '21 at 13:43
  • Also BLDC in the title of your video, does it stand for Brushless Direct Current? Or is it AC being produced? – Adil Mohammed Nov 18 '21 at 13:46
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    BLDC does stand for brushless direct current. The built-in electronic controller that was removed, accepts DC input and produces an output that is AC or similar to AC. The motor itself is more like a permanent-magnet synchronous AC motor and works as an AC generator as shown. –  Nov 18 '21 at 14:22
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Consider a loudspeaker. It's essentially a straightened-out galvanometer: instead of rotating, the coil moves linearly, but the principle is the same. Instead of an indicator, it's coupled to a "cone" that excites sound waves. Apply AC, the cone vibrates, and you hear it. The galvanometer does the same thing, except that the vibrating indicator doesn't make much sound.

John Doty
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Your experiment would work if the galvanometer is properly protected. D'Arsonoval moving coil analog meters are based on a galvanometer with resistors connected in series and/or parallel to limit the galvanometer voltage to say 0.1V and galvanometer current to say 1mA.

Properly buffered by a shunt resistor for an ammeter, which deflects the majority of the current allowing a max of 1mA through the galvanometer, or a series resistor for a voltmeter, dropping the majority of the voltage allowing 0.1V across the galvanometer.

At 50Hz (or 60Hz), the best you would see is a slight quiver at the end of the needle.

AC analog meters add a diode to eliminate the negative portion of the waveform to get a net proportional deflection.

StainlessSteelRat
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  • Oh so you are saying I have a better chance of seeing a deflection with an ammeter/voltmeter. Also what's an analog meter? Is it something used to find current in AC? – Adil Mohammed Nov 17 '21 at 12:51
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    The galvanometer is an analog meter. It is the meter movement for all analog meters (meters with needles that deflect). – StainlessSteelRat Nov 17 '21 at 12:59
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    @AdilMohammed analog meters are the ones with the pointers that move, digital meters display a number like 12.3 – user253751 Nov 17 '21 at 15:40