Question

1) How does an ammeter works and its internal structure.
2) How a voltmeter works and its internal structure.

3) In a circuit with resistors in parallel,how can we control the amount of current that will flow through each resistor?
4) How does a rheostat work?

Answer 1

As we know a word "meter" associated with the measurement. Meter is an instrument which can measure a particular quantity. we know, the unit of current is Ampere. Ammeter means Ampere-meter which measures ampere value. Ampere is the unit of current so an ammeter is a meter or an instrument which measures current.

Working Principle of Ammeter
The main principle of ammeter is that it must have a very low resistance and also inductive reactance. Now, why do we need this? can't we connect an ammeter in parallel? The answer to this question is it has very low impedance because it must have very low amount of voltage drop across it and must be connected in series connection because current is same in the series circuit. Also due to very low impedence the power loss will be low and if it is connected in parallel it becomes almost a short circuited path and all the current will flow through ammeter as a result of high current the instrument may burn. So due to this reason it must be connected in series. For an ideal ammeter, it must have zero impedance so that it has zero voltage drop across it so the power loss in the instrument is zero. But the ideal is not achievable practically.

Depending on the constructing principle, there are many types of ammeter we get, they are mainly -

1) Permanent Magnet Moving Coil(PMMC) ammeter.
2) Moving Iron(MI) Ammeter.
3) Electrodynamometer type Ammeter.
4) Rectifier type Ammeter is a voltage meter, which measures the voltage between the two nodes. We know the unit of potential difference is volts. So it is a measuring instrument which measures the potential difference between the two points. Voltmeter Working Principle of Voltmeter
The main principle of voltmeter is that it must be connected in parallel in which we want to measure the voltage. Parallel connection is used because a voltmeter is constructed in such a way that it has a very high value of resistance. So if that high resistance is connected in series than the current flow will be almost zero which means the circuit has become open.

If it is connected in parallel, than the load impedance comes parallel with the high resistance of the voltmeter and hence the combination will give almost the same the impedance that the load had. Also in parallel circuit we know that the voltage is same so the voltage between the voltmeter and the load is almost same and hence voltmeter measures the voltage. For an ideal voltmeter, we have the resistance is to be infinity and hence the current drawn to be zero so there will be no power loss in the instrument. But this is not achievable practically as we cannot have a material which has infinite resistance.

Classification or Types of Voltmeter

According to the construction principle, we have different types of voltmeters, they are mainly -

1) Permanent Magnet Moving coil (PMMC) Voltmeter
2) Moving Iron (MI) Voltmeter.
3) Electro Dynamometer Type Voltmeter
4) Rectifier Type Voltmeter
5) Induction Type Voltmeter
6) Electrostatic Type Voltmeter
7) Digital Voltmeter (DVM)