Voltmeter Vs. Ammeter

Analog voltmeters and ammeters are modified galvanometers, that can measure current. A galvanometer consists of a coil, wrapped around a magnet, to which a pointer is attached. When electricity passes through the coil, an opposing magnetic field is created, which makes the coil move. The moving coil compresses a spring attached to it, which subsequently deflects the pointer attached to it. Thus, a galvanometer can be calibrated to measure current and voltage, with some modifications.

When exploring the difference between these two electronic instruments, there are two main points of distinction, which are their functionality and internal circuitry. Let us compare voltmeters and ammeters, according to these two main features.

The obvious difference between a voltmeter and ammeter is that the former is used to make an accurate measurement of potential difference, while the latter is used to measure current flowing between two points. They differ in functionality but they measure two aspects of an electrical measurement, which are voltage and amperage.

There is a difference between the ways in which, a voltmeter and an ammeter are connected in a circuit. When measuring the voltage across a resistor or a conductor element, the voltmeter is always connected in parallel. As opposed to this, to measure the current flowing through a circuit element, an ammeter should be connected in series.

Any instrument that makes a measurement of a parameter, does so, with some degree of inaccuracy, as the very act of measuring causes the measurement to be messed up. This could be called a 'Perturbing Effect'. Let us see what is the internal circuit difference between a voltmeter and ammeter and how is the perturbing effect minimized.

As I said before, both the analog voltmeters and ammeters are modified galvanometers. Let us see how they are internally different. A voltmeter is created by adding a high resistance element, in series with the galvanometer coil. The addition of high resistance in series makes the reading of the galvanometer, proportional to the voltage being measured.

It increases the internal resistance of the voltmeter and the resistance used is generally 10 megohm. An ideal voltmeter should have infinite internal resistance so that it does not draw in any current and create a disturbance in measurement. In actual voltmeters, this effect cannot be exactly eliminated but reduced by adding high resistance, in series with the galvanometer.

As opposed to this an ideal ammeter should have zero internal resistance so that it causes no voltage drop across itself, while measuring current. A galvanometer is a natural ammeter and it can function as an ammeter, without any modifications, with the right calibration.

However, to measure large currents, a small shunt resistance is placed, in parallel with the galvanometer, which draws a larger part of the current through it.

As the shunt has very low resistance, connecting it in parallel, with a high voltage source, will short-circuit the instrument. That's why, care should be taken that an analog ammeter is always connected in series.