How to Convert Galvanometer into Voltmeter?

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To Convert the Given Galvanometer of Known Resistance and Figure of Merit into a Voltmeter of Desired Range and to Verify the Same Experiment

Let’s suppose that the water has to be transferred from one tank to another. The energy/push given for the water supply is the potential difference and the measure of the same is done by a voltmeter.

If the waterfalls in minute drops (small electric current), then this water flow is measured by a galvanometer. 

The conversion of galvanometer into voltmeter is done by adding a highly-resistive multiplier in series. 


Galvanometer to Voltmeter Formula

We know that for the conversion of a galvanometer into a voltmeter, a high resistance is required. So, if the resistance of the galvanometer is G and that of the high resistance is R, when they are connected in series, the total resistance of the arrangement becomes the following:

     RSeries    =   G + R    

Now, the galvanometer behaves as a voltmeter. How does this happen? Now, we will look at the same thing in the form of the following experiment.        


Galvanometer to Voltmeter Conversion Experiment

We all know that the potential difference applied across the ends of a conductor is a voltmeter. We can convert the galvanometer into a voltmeter if we know its resistance and the figure of merit.

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Now, let’s perform an experiment to determine the readings we encounter while doing the conversion of galvanometer into voltmeter:


The Objective of the Experiment:

To Convert the Given Galvanometer of Known Resistance and Figure of Merit into a Voltmeter of Desired Range and to Verify the same Experiment.


Apparatus Required for this Experiment:

The following are the instruments required to perform the conversion of galvanometer into voltmeter:

  • A galvanometer

  • Voltmeter of 0-to-3 V

  • A source: battery

  • Two one way keys

  • Two resistance box, one of 10,000 ohms and another of 200 ohms

  • Rheostat: A variable resistor

  • Connection wires

  • A sandpaper

The formula for the series resistance required for the conversion is:

                   R = V/Ig - G


Procedure for the Conversion of a Galvanometer into a Voltmeter:

  • Connect the resistance box in a series combination across the galvanometer and then collect the plugs of resistance R.

  • In the above diagram, A and B are the fixed ends/terminals and C is the variable terminal of the rheostat (a variable resistor).

  • We can see that the galvanometer works as a voltmeter under the range of V Volts.

  • Now, take out the plugs of calculated resistance R from the resistance box.

  • Now, use the key to adjust the movable contact of the rheostat such that the deflection of the galvanometer reaches the maximum ranger.

  • Note the readings of both the galvanometer and voltmeter.

  • Convert the readings of the galvanometer into V or volts.

  • Check if there is a difference in the reading and this difference between voltmeter reading and galvanometer reading may show an error.

  • Now, by moving the variable contact of a rheostat, take six readings covering the range of voltmeters from 0-to-3 V.

Observation of the above Experiment:


Conversion of Galvanometer into Voltmeter Practical Observations

S.No. 

Reading of the Conversion of Galvanometer into Voltmeter

Standard Voltmeter Reading: Vb

Std. Voltmeter Reading: Vb - Va


Deflection

A Potential Difference Va in Volts



1.





2.





3.





4.





5.





6.






Calculation part:

Here, 

The resistance of the galvanometer is = 

The current for a full-scale deflection is = I

Number of divisions on the given galvanometer scale is = n

The figure of merit of galvanometer formula is:

Ig  = nk or k = Ig/n,is the required figure of merit of galvanometer formula.

The resistance in series to be calculated will be used in the following formula:

R = V/Ig - G                        

The final result of the experiment:

We found that there is a minute difference in the value of the actual and the measured one and the conversion is seemingly perfect.


Conversion of Galvanometer into Ammeter Practical Observations

For the conversion of a galvanometer into an ammeter of range ‘I’, we require a shunt resistance and it can be calculated by the following formula:

                               S = (Ig * G)/(I - Ig)

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Where,

S = shunt resistance

Ig = nk  is the needed full-scale deflection of a galvanometer. The unit of the figure of merit of a galvanometer is amp/div.

I = A range of the desired ammeter in mA

The length of the wire required to create a shunt is calculated by:

I = π\[^{r^{2}}\]  S/ρ

Where,

r = radius of the wire calculated by using a screw gauge. This wire is used for making the shunt 

ρ = the resistivity of the given material wire


Conversion of Galvanometer into Ammeter Practical Observations:

  1. The resistance of the galvanometer in …..ohms.

  2. The figure of merit in……..ampere per division.

  3. A number of divisions in a given galvanometer, n…..

  4. The desired range of the current in an ammeter is…...milliamperes.


S.No. 

Ampere Reading ‘I’

Galvanometer Reading (I)’

Current Passing via the Converted Ammeter 

I’ = Ig’ *  (G + S)/S

Error Encountered in the Converted Ammeter:

 (I - I’)

1.





2.





3.





4.





5.





FAQ (Frequently Asked Questions)

Question 1: What is a Galvanometer? Why Does it Become Difficult for a Galvanometer to Function as an Ammeter?

Answer: A galvanometer is an electromechanical device that is used for detecting and indicating an electric current of a smaller magnitude. A galvanometer can function as an actuator by generating a rotary deflection of a pointer in the effect of an electric current flowing via a coil in an invariant magnetic field.


Working principle of a galvanometer: Galvanometers are developed from the observation that the needle of a magnetic compass deflects near a wire through which the current flows.


A galvanometer is a device with high resistance and low current capacity. So, when a large amount of current passes through it as required in an ammeter, the galvanometer may get damaged. So, this is the reason why galvanometer conversion into an ammeter becomes a little tough task.

Question 2: How Does a Galvanometer Work? How Sensitive are Galvanometers?

Answer: If we connect a galvanometer in a Wheatstone’s bridge circuit, the pointer in the galvanometer shows null deflection, which means that no current flows through the device. However, if the pointer deflects to the left or right that means the current starts flowing through it and the direction of deflection depends on the direction of the current. 


A galvanometer is a very sensitive device that can detect the small currents and it can be easily converted into an ammeter and a voltmeter. However, the conversion of galvanometer into ammeter and voltmeter requires a good study on interconnection because galvanometers are very sensitive instruments for detecting the current.