To Convert The Given Galvanometer (of Known Resistance And Figure of Merit) into a Voltmeter of Desired Range and To Verify the Same
To convert the given galvanometer (of known resistance and figure of merit) into a voltmeter of desired range and to verify the same.
A weston type galvanometer, a voltmeter of 0-3 volts range, range a battery of two cells or battery eliminator, two (10,000 Ω and 200 Ω) resistance boxes, two one way keys, a rheostat, connecting wires and a piece of sand paper.
- Calculate the value of series resistance R to be connected in series with the galvanometer for the given range V (say 3 volts).
- Connect a resistance box in series with the galvanometer and take out the plugs of resistance R. Now the given galvanometer is ready for use as a voltmeter of range V volts.
- For verification make the connections as shown in the circuit diagram. Here AB is a rheostat being used as a potential divider. A and B are the fixed terminals and C is the variable terminal of the rheostat.
- Take out the plugs of calculated resistance R from the resistance box. Insert the 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 of the rheostat and take at least five observations covering the whole range of the voltmeter i.e., 0-3 volts.
- Record your observations.
As the difference in actual and measured value of potential difference (as recorded, in column 4) is very small, the conversion is perfect.
- All the connections should be neat, clean and tight.
- The e.m.f. of the cell or battery should be constant.
- The ammeter used for verification should preferably be of the same range, as the range of conversion.
- The diameter of the wire to be used for shunt resistance, should be measured accurately.
- Length of shunt wire used should be neither too small nor too large.
- The resistance box should be a high resistance one.
- The voltmeter used for verification should preferably be of the same range, as the range of conversion.
- Value of required series resistance should be calculated accurately.