Aim : To convert the given galvanometer into a voltmeter of desired range and to verify the same .
Apparatus : A Weston type galvanometer , a voltmeter of 0-3 V range , a battery eliminator , two resistance boxes , the one way key , a rheostat , connecting wires .
Theory : the series resistance required for conversion ,
Where V is the range of conversion .
- Calculate the value of series resistance R to be connected In series with the galvanometer for the given V .
- Connect a resistance box in series with the galvanometer and take out the plug of R . The galvanometer is now ready for use as voltmeter of range V volts .
- For verification , make the connection as shown in diagram . A and B are the fixed terminals and C is the variables terminals .
- Take out the plugs of calculated R from the resistance box . Insert a key K and adjust the movable contact of the rheostat so that deflection in the galvanometer becomes maximum .
- Note the readings of voltmeter and galvanometer . Convert the galvanometer reading into volts.
- Find the difference , if any between the readings of voltmeter and galvanometer . The difference gives the error .
- Move the variable contact C and take five observation covering whole range of voltmeter .
- Record your observation.
As the difference in actual and measured value of potential difference is very small , the conversion is perfect .
1. Length of the shunt wire should be neither too short nor too long .
2. The voltmeter used for verification should preferable be of the same range as the range of conversion .
3. Value of required series resistance should be calculated accordingly .
Sources of error
The connection may be loose .
The emf of the cell or battery may be variable .
Circuit diagram :-
Resistance of given galvanometer (G) 60 ohm
No of division (n)=10
Current (Jg) =nk=0.00028A
Range of conversion = V
Resistance to be placed in series = Rs
Rs =3/0.00028 -60.7
Rs = 10653.5 Ohm
% error = 3-2.8/3 x 100
=0.2/3 x 100
Why is a Galvanometer not suitable to work as an ammeter .
A galvanometer has more resistance and less current . carrying capacity from those required in ammeter . It will damage if large current flows through it .
What is a shunt ?
A small resistance connected in parallel with a galvanometer is called shunt .
Why is a galvanometer not suitable to work as voltmeter ?
A galvanometer has less resistance and more current carrying capacity from those required by a voltmeter .
How is a galvanometer converted into a voltmeter ?
A galvanometer is converted into a voltmeter by connecting a high resistance in series with the galvanometer coil.
What happens when a voltmeter is converted in series in a circuit ?
The voltmeter cannot measure the potential difference in the circuit because overall resistance of circuit increases .