# To Convert the Given Galvanometer (of Known Resistance and Figure of Merit) Into An Ammeter of Desired Range and to Verify the Same

## Aim/Aim of Experiment

To convert the given Galvanometer of known resistance and figure Of Merit) into an Ammeter of desired range and to verify the same.

## Apparatus/Material Required

• A Weston Type Galvanometer
• A Ammeter (0-3 A)
• A Battery (Two Cells)
• Two Resistance boxes (10,000 ohm & 200 ohm)
• A Rheostat
• Two One-way Keys
• Connecting wires
• A piece of Sand paper.

## Theory

A Galvanometer is a sensitive device which can detect the presence of very small. For measuring current of the order of an ampere, a low resistance called shunt resistance S is connected in parallel across the galvanometer having resistance G.

If I is the total current in the circuit for full scale deflection, then the current (I – Ig) passes through S, where Ig is current that flows through the galvanometer for full scale deflection. Since G and S are parallel to each other therefore, the potential difference across both are same, hence,

IgG=(I-Ig)S

So, the shunt resistance required for conversion, S = IgG/I-Ig,

where I is the range of conversion.

## Procedure

1. Count the total number of divisions on either side of zero of the galvanometer scale (say n).
2. Calculate the current Ig or full scale deflection in the galvanometer i.e. Ig = nk, where k is the figure of merit of the galvanometer.
3. For conservation into ammeter, calculate the value of shunt resistance (S) by using the formula S=IgG/I-Ig. (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, eureka, etc., of a suitable diameter and length).
4. For calculate the length of wire (l), cut a length of the wire 2 cm more, than calculated the value (l) by using the formula [l=Sπr2/ρ].
5. Now 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).
6. Connect the apparatus or instruments the same as shown in circuit diagram.
7. To observe maximum and minimum deflection in the galvanometer, Insert the key and adjust the rheostat.
8. Note down the reading on the galvanometer scale and also corresponding reading from the ammeter.
9. Record the observations.

## Calculations

• The Galvanometer Resistance (G) = 80 ohm.
• Figure of Merit of the Galvanometer (k) = 1.9×10-5 Amp/div.
• Number of divisions in Galvanometer scale (n) = 30 division.
• Current for full scale division (Ig) = Ig=nk = 57×10-5A.
• Range of conversion (I) = 3A.
• Shunt resistance (S) = S=IgG/I-Ig = 0.015 ohm.

## Verification

Least count of the galvanometer converted into an ammeter = I/n = 0.1A.

## Observation

Table for Verification for converted Ammeter:

## Result

The result of this conversion is as the difference (Error) in actual value and measured value of current (as recorded, in column 4) is very small or about zero, so here the conversion is perfect.

## Precutions

• Ensure that all the connections should be neat, clean and tight.
• 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, the length shunt wire must be correct, length of shunt wire should be neither too small nor too large.

## Sources of Error

• The connection may be loose.
• The e.m.f. of the cell or battery may be variable.
• Length of shunt wire may not be correct.

## Viva Voice Questions with Answers

1. Define the Galvanometer.

Answer: Voltmeter is a device (instrument) used for detecting feeble electric currents in circuits.

2. What is Weston Galvanometer?

Answer: The Galvanometer which has pivoted coil and used in the laboratories is known as Weston Galvanometer.

3. Define figure of merit of a galvanometer.

Answer: The quantity of current required to produce a deflection of one division in the galvanometer, is called figure of merit of the galvanometer. It is represented by the symbol k. Its unit is ampere per division.

4. Define the resistance of a galvanometer?

Answer: The resistance of the coil of a galvanometer, is called the resistance of the galvanometer. It is represented by the symbol G.

5. Under what conditions, G = S?

Answer: G = S, only when series resistance R is very high.

6. What is an ammeter?

Answer: An ammeter is a device (instrument) for measuring large electric currents in circuits.

7. What are the required properties of an ammeter?

Answer: An ammeter must have a very small resistance (zero, if possible) and a large current carrying capacity.

8. Why is a galvanometer not suitable to work as ammeter?

Answer: A galvanometer has more resistance and less current current-carrying capacity from those required by an ammeter. It will damage when large current flow through it.

9. How is a galvanometer converted into an ammeter?

Answer: A galvanometer is converted into an ammeter by connecting a low resistance in parallel with the galvanometer coil (this parallel low resistance is called shunt).

10. How the low resistance is parallel (shunt) gives required properties to the galvanometer?

Answer: The shunt reduces the overall resistance of the ammeter (converted galvanometer) and increases its current-carrying capacity.

11. What is the order of resistance of an ammeter?

Answer: The ammeter resistance is nearly equal to the shunt resistance.

12. What do you understand by range of an ammeter?

Answer: It is the maximum value of the current which an ammeter can measure.

13. What is full name of an ammeter?

Answer: Full name of an ammeter is ampere-meter.

14. Define shunt? State its S.l. unit. Answer. A small resistance connected in parallel with a galvanometer is called shunt. Its S.l. unit is Olim.

15. How are ‘figure of merit’ and ‘current sensitivity’ related to each other?