Voltmeter-Definition, construction, Practice problems, FAQs

Shriya buys two bulbs from the hardware store. She looks carefully at the bulbs and notices something like written across them. In order to understand what “means, she does a simple Google search and comes across a term named “ safety rating”. She then concludes that every electrical equipment has a safe value of voltage or current that it can withstand before fusing. For instance, a bulb which has written across it means that it can withstand up to volts across it. In other words, for an electrical appliance, if a voltage above a certain value is passed, it turns out to be harmful for the appliance. In order to have control over the voltage drop occurring across an appliance, Shriya needs to use an equipment called voltmeter. The first digital voltmeter was invented by Andrew Kay in 1954. In this article, we will explore voltmeters in detail.

Table of contents

• What is a voltmeter?
• Converting a galvanometer into a voltmeter
• Practice problems
• FAQs

What is a voltmeter?

Voltmeter is a device which is used to measure the voltage or potential difference that has occurred across an appliance. Voltmeter is always connected in parallel.

An ideal voltmeter is one which has infinite resistance. Consider a resistor connected in the circuit. If denotes the current flowing in the circuit, then

  indicates the resistance of the voltmeter. 

Converting a galvanometer into a voltmeter

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.

Practice problems

Q. In the circuit diagram shown below, the emf of the cell is The resistance of the voltmeter connected is Find out the voltmeter reading?

(a) (b) (c) (d)

A.c

The total resistance of the circuit,

The net current flowing,

So the current flowing through each of the resistances connected in parallel would be

Potential difference across the resistance

Q. Two resistors having values and this combination is connected to a battery having emf and a resistor. Calculate the potential difference across the terminals of the resistor? 

(a) (b) (c) (d)

A.a

Given, and  

The equivalent resistance of this parallel combination,

Now the net resistance of the circuit,  

The current flowing in the circuit, 

Let and be the currents flowing through and respectively. Then

Solving we get 

Now potential difference across the resistor is

Q.and are 3 voltmeters whose resistances are and respectively. When a potential difference is applied between the points and their respective voltmeter readings are and Then which of the following relations is true?

(a) (b) (c) (d

 A.a

Given, ,

Now

and

and

.

Q. A galvanometer of internal resistance requires for a full scale deflection. To convert this into a voltmeter that shows full scale deflection of we need to connect a resistance of 

(a) in series with it (b) in series with it 

(c) in series with it (d) in parallel with it

A. b

Given

 Substituting the values, we get

To convert a galvanometer into a voltmeter, we need to connect a high resistance in series.

FAQs

Q. What is the difference between an ammeter and galvanometer?
A. A galvanometer is used to measure small currents of the order of milliampere while an ammeter can measure large currents.

Q. An ideal ammeter has zero resistance. Why?
A. An ideal ammeter has zero resistance so that the actual current reading of the branch in which current is to be measured does not change. 

Q. An ideal voltmeter has infinite resistance. Why?
A. Ideally there should be no current through the voltmeter and the entire current must pass through the element across which voltage is to be measured, so that we get the actual value of voltage. So, an ideal voltmeter has infinite resistance.

Q. Which other instrument can be used to measure potential difference?
A. Potentiometer can also be used to measure the potential difference.