Convert a galvanometer into an ammeter of range 0–1.5 ampere.

Apparatus

• Galvanometer
• ammeter
• voltmeter
• accumulator
• resistance box
• plug key
• connecting wire
• Shunt wire and screw gauge

Theory:

A Galvanometers having a resistance G give full scale deflection when a current I_gis passed through it. It can be converted into an ammeter of range (0 – 1) by connecting a suitable small resistance S in parallel with it (Fig.5.a).The value of the shunt resistance S is connected such that when this parallel combination of galvanometer and shunt resistance S is connected in series with a circuit caring a current I, it allows a current I to pass through the galvanometer and the rest of the current (I – I_g ) through the attached shunt S.

The value of the shunt resistance S can be collected by potential different across the tow branch of the circuit (Fig.5.a)

L G=(1-)S    or                            s=

Thus a wire having resistance S when connected in parallel with the galvanometer converted it into an Ammeter of range (0. I).

Procedure

• Resistance of the galvanometer by half deflection method.
• Arrange the apparatus as shown in Fig.5.a. Apply a suitable high resistance by the resistance box RB and close the key K₁ keeping the key K₂open. Adjust the resistance from the resistance box RB so as to get a large deflection. Close K₂ also and adjust the shunt resistance S so that the deflection of the galvanometer is reduced to half. The resistance G of the galvanometer is equal to the shunt resistance S.
• Current I_g for full scale deflection..

Measure the EMF (E) of the accumulator by a voltmeter. Disconnect the shunt resistance from the apparatus shown in Fig.5.a Close the key K₁and apply shunt resistance S is connected such that when this parallel combination of galvanometer and shunt resistance S is connected in series with a circuit caring a current I, it allows a current I to pass through the galvanometer and the rest of the current.

• Consulting the table of the Constance (Table 10, page 63) find the length of a copper wire of which has a resistance equal to the shunt resistance S.

Cut off a piece of wire a little large then the calculated length. Mark two points neat calculated distance from each other leaving spare length at each end of a wire.

Verification

Connect this piece of wire in parallel with the galvanometer having a resistance G give full scale deflection when a current I_g is passed through it. It can be converted into an ammeter of range (0 – 1) by connecting a suitable small resistance S in parallel with it (Fig.5.a).The value of the shunt resistance S is connected such that when this parallel combination of galvanometer and shunt resistance S is connected in series with a circuit caring a current I.

Observation & Calculation.

• Resistance of Galvanometer:

Zero reading of the galvanometer= ____________

No of Obs.	Resistance R (ohms)	Deflection of Galvanometer		Half Deflection (div)	Shunt resistance      S (ohms)	Resistance  Of  galvanometer  (ohms)
		Observed (div)	Corrected 𝛉(div)
1 2 3

Mean resistance of the galvanometer g = ________________Ohms.

• Current for full scale deflection:

Resistance used (R)=____________ other.

Total number of division on the galvanometer scale = n = ____________ div

EMF of the cell = E = ____________ volts.

Deflection produced = = __________ A

Current in the circuit = I = _________A

Current for full scale deflection = I_g =

• Calculation of shunt resistance:

Max. Range = I = 1.5 A                      Shunt resistance = S= ___________ ohms.

• Calculation of length of shunt wire:

Diameter of copper wire = ___________ ohms.

Viva voice

1. What is meant by an ammeter?

Ans. It is an electrical instrument used for measuring current in amperes.

• How is an ammeter connected in a circuit and why?

Ans. An ammeter is connected in series with the circuit so that current passing through the circuit should also though the ammeter.

• Can you use a D.C ammeter for measuring alternating current?

Ans.  NO.

• How will you convert a galvanometer into an ammeter?

Ans. A suitable low resistance should be connected in parallel with the galvanometer in order to convert it into an ammeter.

• Why an ammeter should have a low resistance?

Ans. An a meter should have a low resistance so that it may not change the current which it is required to measure

• What is ammeter?

Ans. It is low resistance galvanometer which is used to measured current in amperes.

• How will you convert galvanometer into an ammeter?

Ans. Galvanometer can be converted into an ammeter by connecting a suitable low resistance (i.e., shunt)

• Why should be the ammeter be always connected is series in the circuit?

Ans. It is connected in series in the circuit, because to measure the

• What will ……….connected in parallel in the circuit?

Ans. If an ammeter is connected in parallel to a circuit, it will not measure the current and will damage due to flow of excess current through it.

1.  Why should an ammeter have a very low resistance?

Ans. Ammeter should have a very low resistance because it may not change the current which is t be measured.

1. Why can alternating current (A.C) not be measured by ordinary ammeter?

Ans. Since direction of A.C changes continuously, and ordinary ammeter shows its average value which is equal to zero, Hence pointer of ammeter will remain at rest.

1. Is there any method to measure A.C?

Ans. Yes, A.C can be first converted into D.C by a rectifier and than it can be measured by D.C ammeter.

1. How can we increase the range of the ammeter?

Ans. We can increase the range of the ammeter by decreasing the value of shunt resistance.

1. What id SI unit of charge and defines it?

Ans. SI unit of charge is coulomb and can be defined as the quantity of charge flowing in a will in one second when the current in the wire is one ampere.

1. What is difference between D.C and A.C?

Ans. Direct current (D.C) flows in one direction only and its magnitude remains constant where as alternating current (A.C) changes periodically and its magnitude also changes continuously.

1. What is open circuit?

Ans. A circuit with infinite resistance or a broken circuit is known as open circuit.

1. What is short circuit?

Ans. A circuit with zero (or nearly zero) resistance is known as short circuit.