Video: Eq17S1-Physics-Q04

Explain why the zero position of a moving-coil galvanometer is at the middle of its scale.

03:17

Video Transcript

Explain why the zero position of a moving coil galvanometer is at the middle of its scale.

To get started, let’s consider first a bit about what this galvanometer is for and also how it works. A galvanometer is a device used to measure electrical current. And a moving coil galvanometer is designed in a particular way to do this. At the center of a moving coil galvanometer is a metal core wrapped many times in wire. This wire wrapped core sits between the poles of a permanent magnet which creates a magnetic field that this core experiences.

When current, coming from the circuit that the galvanometer is measuring, flows through this wire and around the loops of this core, in the presence of the magnetic field, this coil of wire begins to experience a torque. That is, it starts to want a twist. Attached to a point along the axis of rotation of the coil is a needle. And as the coil turns, this needle moves as well. It’s the position of this needle on a scale which indicates the current running through the wire and, therefore, running through the circuit. This is the mechanism by which a moving coil galvanometer measures current.

If we looked closely at the scale though, we would see that the marking for zero current in the circuit is at the middle of the scale. The question is, why is that so? Why is the zero point in the middle? After all, if we consider other measurement devices, like say a ruler, we’re more used to seeing zero appear at one end of the scale and then values move up from there. The fact that we have a zero at the middle of our galvanometer scale seems to indicate that we could have a negative value for current. And in fact, that idea gets that just why it is that the zero was in the middle of the scale rather than at one end.

Let’s say we were to turn the current in this circuit off completely, so that no current runs through the loops of our galvanometer. And the needle moves to exactly zero. If we were to send in a much larger current than before, call it 𝐼 two, then our coil of wires in the core of our galvanometer would experience a torque, would rotate, and our needle would reflect that rotation on the scale.

This tells us that if we send current 𝐼 two in on this wire and it comes out on this wire, then our coil will rotate clockwise. And our needle will indicate a current in the direction to the right of zero. But now, here’s a question. What if we use the same magnitude of current in our a circuit, but we had it moving in the opposite direction, in where it’s currently out and out where it’s currently in?

If we made that change reversing the direction of the current flowing through our wire, then once more the core of our galvanometer would experience a torque. It would rotate. And the needle would reflect that rotation on the current scale. And this time, we see the needle has gone past zero into what would seem to be negative values. We’ve hit on then why it is that the zero position of our galvanometer is at the middle of its scale. It has to do with the fact that current in our circuit can be moving in either one of two directions.

By having the zero position in the middle of our scale, our galvanometer is able to tell us something about current direction as well as magnitude. Here’s what we could say then about the reason for the position of this zero point. We can say that, depending on the direction of the current through the galvanometer coil, the pointer in the coil can deflect either clockwise or counterclockwise. This is the reason behind the zero position of the scale being at its middle.

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