Ammeters and voltmeters are based on the galvanometer, a device that is used to detect small currents. Galvanometers that are calibrated to measure currents of different amounts are called ammeters. When they are modified to measure voltages, they are called voltmeters.
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The galvanometer is named after Italian physicist Luigi Galvani. While he was dissecting a frog's leg, he found that the leg twitched when two different metals touched it. Alessandro Volta improved on Galvani's theory and findings, and this discovery also led to the invention of chemical cells and batteries.
A galvanometer is constructed from coils on springs, a needle and a stationary magnet. When a current is passed through the coils, the needle is deflected. This deflection is proportional to the amount of current.
Galvanometers are modified to be either ammeters or voltmeters.
Ammeters measure current flowing in a circuit. They are galvanometers that are placed in parallel with a resistor. The resistor in this capacity is referred to as a shunt. Its value is chosen to be very small, in order to limit the amount of internal resistance of the galvanometer.
Galvanometers as ammeters can directly measure DC current but to measure AC, a diode is placed inside the device. The purpose of this diode is to limit the current flow to one direction. Without it, the needle would fluctuate back and forth or not move at all, and it would not be possible to take a reading.
Voltmeters measure the potential difference or voltage in a circuit, and are galvanometers that are placed in series with a resistor. Ideal galvanometers used this way must have an infinite resistance, and this is approximated having the value of the resistor be very large.
Once again, a diode must be placed inside in order for it to measure AC currents.
Ammeters vs. Voltmeters
Ammeters and voltmeters are the opposite of each other not only in terms of construction, but of circuit placement.
Ammeters must have current passing through them in order to measure it, and so must be placed in series inside of a circuit. Polarity must be observed, and their positive and negative leads must be connected to the positive and negative sides of the circuit, respectively. Also, their internal resistance must not be large or it will block too much current, and this will affect the way the circuit functions and hence the reading.
Voltmeters should not have any current passing through them, and so are placed in parallel with the circuit they are to measure. Their leads are placed on each side of the component of interest, and as with ammeters, polarity must be observed. If their internal resistance is too small it will not block enough current, and this will affect the measurement.
Multimeters are devices that can function as ammeters or voltmeters. They also perform as ohmmeters, which means they can measure resistance. They have a knob or button which allows them to switch between the various functions. Digital multimeters are more commonly used than analogue.
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