Sachin buys three bulbs from the hardware store. Let’s name these bulbs and He connects them in parallel to an AC source using wires and finds that the bulb fuses after some time. He is confused why this happens; so he does a little research on the internet and learns that any electrical appliance can only withstand a certain amount of current. For instance, let indicate the safe current that can be passed through bulb . Let be the safe current that can be passed through bulb It is clear that is greater than But how to know their values before allowing them to pass through the bulb? A device called ammeter is going to help Sachin know this. Similar to the above, each electrical equipment has a safe voltage that it can withstand before it fuses. For instance, a bulb rated means that it can safely operate on a source voltage up to How to know this value? A device called voltmeter is needed for this. In this article, we will explore ammeter and voltmeter in detail.
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Ammeter is a device used for measuring currents. On the other hand, a galvanometer is used to measure smaller currents. An ammeter is always connected in series in a circuit. It has a needle which deflects on a graduated scale. The scale is graduated in amperes () or milliamperes ( An ideal ammeter is one which has zero resistance.
Let us consider the following circuit where indicates the resistance of the ammeter and indicates the external resistance. The current flowing in the circuit can be written as,
. by definition of an ideal ammeter.
Then
I.e. the reading would be the same as when a resistor is connected. From equation it is clear that lower the resistance of the ammeter, more accurate will the reading be.
A voltmeter is a device used for measuring potential difference across an appliance. A voltmeter is always connected in parallel in a circuit. It has a needle which deflects on a graduated scale. The scale is graduated in volts () or millivolts ( An ideal voltmeter is one which has infinite resistance. Higher the resistance of the voltmeter, more accurate would be its reading.
Ammeter | Voltmeter |
1)It measures current flowing in the electrical equipment. |
It measures voltage across the electrical equipment. |
2) It is connected in series with the electrical equipment in which the current flowing has to be measured. |
It is connected in parallel across the electrical equipment in which voltage has to be measured. |
3) Ideal ammeter has zero resistance | Ideal voltmeter has infinite resistance |
A galvanometer can be converted into an ammeter by connecting a low resistance called shunt in parallel with it; so that the majority of the current will be allowed to pass through the shunt while only a small portion of the current will pass through the galvanometer
Let be the current in the circuit and be the full scale deflection of the galvanometer.
is the current that will pass through the shunt. The combination of galvanometer and shunt will behave as an ammeter. Let be the value of shunt resistance and indicate the galvanometer resistance. Since they are connected in parallel, their voltage drop would be the same, meaning the product of their respective currents and resistances are equal.
I.e,
The above equation gives the value of shunt resistance which needs to be connected in parallel with the galvanometer to be able to measure current.
The effective resistance of the ammeter so formed is given by .
A galvanometer is converted into a voltmeter by connecting a high resistance in series with it. In the following diagram, let indicate the resistance of the galvanometer. The resistor to be used in series is Let be the voltage across the combination, then by Ohm’s law,
The above equation gives an idea about the resistance that needs to be connected in series with the galvanometer to convert into voltmeter.
Q1. Construct a voltmeter from a galvanometer whose resistance is given by and a full scale deflection of into a voltmeter with a maximum range of ?
Answer: Given , galvanometer resistance ,
Let be the resistance that needs to be connected in series. Then,
Q2. Calculate the value of shunt resistance required to convert a galvanometer into an ammeter with a range of to
(a) (b) ( c ) (d)
Answer: b
Given, full scale deflection of the galvanometer,
The value of shunt resistance that needs to be connected is
Solving, we get
Q3. A bulb having a resistance of is connected to an external resistor having a resistance of The emf of the battery supplying current is Calculate the current flowing in the circuit?
(a) (b) ( c ) (d)
Answer: b
Total resistance of the circuit
Current flowing in circuit
Q4. A cell having an emf of supplies a current of to one of the resistor in a circuit having resistance A voltmeter is connected in parallel across this resistor. Find its reading?
(a) (b) ( c ) (d)
Answer: a
Given, and
By Ohm’s law, .
Question 1. What is the typical range of an ammeter?
Answer: An ammeter typically measures to It carries about 30 divisions on its scale.
Question 2. What is the least count of a voltmeter?
Answer: The least count or smallest possible measurement that can be made with a voltmeter is generally
Question 3. What is the SI unit for potential difference?
Answer:The SI unit for potential difference is volt.
Question 4. Write one use of a moving coil galvanometer?
Answer: It is used to measure small currents.