Aim
To convert the given galvanometer (of known resistance and figure of merit) into an ammeter of desired range and to verify the same.
Apparatus
A weston type galvanometer, an ammeter of 0-3 A. 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.
Theory
to-convert-the-given-galvanometer-of-known-resistance-and-figure-of-merit-into-an-ammeter-of-desired-range-and-to-verify-the-same-1
Circuit diagram
to-convert-the-given-galvanometer-of-known-resistance-and-figure-of-merit-into-an-ammeter-of-desired-range-and-to-verify-the-same-2
Procedure
  1. Count the total number of divisions on either side of zero of the galvanometer scale. Let it be n.
  2. Calculate the current (Ig) for full scale deflection i.e., Ig = nk.
  3. Calculate the value of shunt resistance, for conversion into ammeter, using the formula,
    to-convert-the-given-galvanometer-of-known-resistance-and-figure-of-merit-into-an-ammeter-of-desired-range-and-to-verify-the-same-3
    where I is the range of conversion.
    (The value of shunt resistance S is usually very small and a resistance box of that range is not available. Such small resistances are obtained by taking wires of copper, constantan, manganin, eureka, etc., of a suitable diameter and length).
  4. Cut a length of the wire 2 cm more than the calculated value l. Mark two points on the wire, one cm away from each end. Connect this wire to the two terminals of the galvanometer such that the marked points are just outside the terminal screws. This galvanometer with shunt wire, will now work as an ammeter of range I.
  5. Make the electric connections as shown in circuit diagram.
  6. Insert the key and adjust the rheostat so that the galvanometer shows nearly maxi-mum deflection.
  7. Note the reading on the galvanometer scale and also corresponding reading on the ammeter.
  8. Record your observations.
Calculations
to-convert-the-given-galvanometer-of-known-resistance-and-figure-of-merit-into-an-ammeter-of-desired-range-and-to-verify-the-same-4
Verification 
to-convert-the-given-galvanometer-of-known-resistance-and-figure-of-merit-into-an-ammeter-of-desired-range-and-to-verify-the-same-5
to-convert-the-given-galvanometer-of-known-resistance-and-figure-of-merit-into-an-ammeter-of-desired-range-and-to-verify-the-same-6
Result
As the difference in actual and measured value of currents (as recored, in column 4) is very small, the conversion is perfect.
Precautions
Same as given in Experiment 8.