By connecting resistors to this galvanometer in different ways, you can use it as either a voltmeter or ammeter to measure a broad range of voltages or currents. A galvanometer can function as a voltmeter when it is connected in series with a large resistance R. The value of R is determined by the maximum voltage that will be measured. The total resistance must be:. R is so large that the galvanometer resistance, r, is nearly negligible.
This voltmeter would not be useful for voltages less than about half a volt, because the meter deflection would be too small to read accurately. For other voltage ranges, other resistances are placed in series with the galvanometer. Many meters allow a choice of scales, which involves switching an appropriate resistance into series with the galvanometer.
The same galvanometer can also function as an ammeter when it is placed in parallel with a small resistance R , often called the shunt resistance. Since the shunt resistance is small, most of the current passes through it, allowing an ammeter to measure currents much greater than those that would produce a full-scale deflection of the galvanometer.
Suppose, for example, we need an ammeter that gives a full-scale deflection for 1. Since R and r are in parallel, the voltage across them is the same. Null measurements balance voltages so there is no current flowing through the measuring devices that would interfere with the measurement.
Standard measurements of voltage and current alter circuits, introducing numerical uncertainties. Voltmeters draw some extra current, whereas ammeters reduce current flow. Null measurements balance voltages, so there is no current flowing through the measuring device and the circuit is unaltered. Null measurements are generally more accurate but more complex than standard voltmeters and ammeters. Their precision is still limited. When measuring the EMF of a battery and connecting the battery directly to a standard voltmeter, as shown in, the actual quantity measured is the terminal voltage V.
Voltmeter Connected to Battery : An analog voltmeter attached to a battery draws a small but nonzero current and measures a terminal voltage that differs from the EMF of the battery.
Note that the script capital E symbolizes electromotive force, or EMF. Since the internal resistance of the battery is not known precisely, it is not possible to calculate the EMF precisely. The EMF could be accurately calculated if r were known, which is rare. However, standard voltmeters need a current to operate. A potentiometer is a null measurement device for measuring potentials voltages. A voltage source is connected to resistor R, passing a constant current through it.
There is a steady drop in potential IR drop along the wire, so a variable potential is obtained through contact along the wire. An unknown emf x represented by script E x connected in series with a galvanometer is shown in. Note that emf x opposes the other voltage source. The location of the contact point is adjusted until the galvanometer reads zero. Since no current flows through the galvanometer, none flows through the unknown EMF, and emf x is sensed.
Potentiometer : The potentiometer is a null measurement device. A voltage source connected to a long wire resistor passes a constant current I through it. An unknown EMF labeled script Ex is connected as shown, and the point of contact along R is adjusted until the galvanometer reads zero. The unknown EMF is thus proportional to the resistance of the wire segment. I've done that, and you burn out a fuse, you gotta go replace the fuse and it's a pain. So don't hook up your ammeter in parallel.
What about voltmeters? Why do we hook those up in parallel? Well, a voltmeter is hooked up in parallel because we want to know the voltage across a circuit element, so on either side.
Voltage, remember, is defined to be the difference between electric potential at two points in space. It makes no sense to ask what's the voltage through a certain point in a circuit. You can ask what current flows through that point in the circuit. But asking what the voltage is at a particular point in a circuit makes no sense.
The only thing that would make sense is asking what's the voltage across two points in a circuit. So I can ask what's the voltage between this point and that point, that makes sense, or I can ask what's the voltage between this point and that point, that makes sense. But asking what's the voltage at a point or through a point, makes no sense. That's what current is. Current flows through a point, voltage is across two points.
The difference in electric potential between two points. That's why we hook up voltmeters in parallel and because we hook up voltmeters in parallel, voltmeters have to have a huge resistance.
Sometimes on the order of hundreds of thousands of ohms or even millions of ohms. So this can be big, big number of ohms. And the reason is, think about it, again our key idea is that we don't want to disturb the thing we're measuring. I'm measuring the voltage across this resistor. If I were to hook up a voltmeter with very little resistance, I just told you what would happen. This current that's flowing out of the battery, would all try to go through this voltmeter.
Not only would it try to mess up the voltmeter, but that's current that's not flowing through R three anymore, and so I wouldn't get a correct reading for the voltage through R three. So we want to make sure our voltmeter has a big resistance so that yes, technically a very, very small amount of current, maybe a milliamp, will flow through this voltmeter, because it's gotta take a reading.
But, we want as small amount as possible, because we want to keep this current flowing through R three the same as it was before we were measuring it, because I know v equals IR. And if I can measure this voltage across here, I want to make sure the current's the same, or I won't be getting an accurate measurement for the voltage. You could ask what would happen if we did hook the voltmeter in series instead of parallel. Voltmeters have a huge resistance, so if I stuck that here, the voltmeter has a huge resistance, you wouldn't break it, it's just that, think about what the current's gonna do.
Current comes out of this battery, it's got a choice, it can go up here through R three and the voltmeter or through R one and R two. I said the voltmeter has hundreds of thousands, even millions of ohms, so this current's just all gonna go this way.
Forget that. It's gonna skip this entirely. If you hook up a voltmeter in series instead of in parallel, you just kill off any current through this portion of the circuit that the voltmeter was hooked up in.
You probably won't break it, so it's not as delicate as the ammeter, but you still mess up your measurement because it wasn't designed to be used that way. So remember voltmeters are hooked up in paralled to the circuit element that you want to determine the voltage across. But ammeters are connected in series to the circuit element that you want to measure. And if you're sitting there thinking, "Pfft, I'm never gonna hook up my ammeter in parallel.
So if you're sitting there all day measuring current with your ammeter setting. Everything's going well. And then you go to measure a voltage, but you forget to switch the dial to volt instead of amps, you'll be hooking up an ammeter in parallel erroneously.
How is the actual measurement of voltage done in most voltmeters? I suppose they don't count electrons and divide by the resistance? Old meters applied the voltage directly to the coil of a meter movement. These had substantially lower input resistance even though the coil was wound with as many turns of thin wire as possible.
I agree with starblue that conventional old movement voltmeters actually measure the current. There the ballast resistor acts as a voltage-to-current converter.
Terrance carrington Terrance carrington 1. OP is asking about the working of voltmeter. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password.
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