How milliammeter can be used as a voltmeter?

Answer 1

A milliammeter is a sensitive instrument placed in series in a circuit and is calibrated to measures current going through it, and therefore the whole circuit, without presenting enough resistance to affect the current going through it or reducing the voltage available to the rest of the circuit.

A voltmeter is a milliammeter used in parallel to the circuit. Since the milliammete has very little resistance, if it is used this way it will present as a short to the rest if the circuit, not give a valid reading, and probably burn out. To use a milliammeter as a voltmeter, a significantly large resistance needs to be placed in series with the milliammeter. If the resistance is large enough, but not too large, it will prevent the meter from being a short, not affect the circuit load but still allow enough current to go through the meter to get a reading. If you want to measure AC (alternating current) you will also have to include a rectifier circuit. To be useful as a voltmeter, the meter will need to be calibrated, a tedious job since the meter responses probably are not linear.

Modern multimeters are available that will act as a voltmeter with several range setting. You can get them at any electronic or tool store such as Sears, Harbor Freight, or Radio Shack. Even the cheap 10 dollar ones will do a good job for most purposes although professional models that do so much more and with greater precision can be really expensive.