Determine the resistance of galvanometer by half deflection method.

Apparatus

• Galvanometer
• Cell
• High
•  Resistance box
•  Frictional resistance box
•  Two keys and connecting wires.

Theory

In half deflection method the galvanometer is connected to a cell through a high resistance R and its deflection is noted. The galvanometer is then shunted by connected a fractional resistance box across it and the shunt resistance is adjusted to reduce the deflection to half of its original value. It means that half of the total current now passes through the galvanometer and the other half flows through the shunt and the resistance of the galvanometer is equal to the resistance of the shunt.

The total current I through the circuit before shunting the galvanometer is given by

I =

Where E is the E.M.F of the cell and G is the galvanometer resistance.

The total current after shunting     = I₁ =  =

The current  through the galvanometer after shunting is given by

I₄ = I_{1  =} ×  =

As  = ½ I by shunting the galvanometer, therefore

                         =

2S(R+G) = R (G+S) = GS or2RS + 2GS + RG + RS + GS

Or        G+

If the series resistance R is very large as compared with the shunt resistance S, the S can be neglected in the denominator in the above equation and we have

G= =S

Procedure

1. Draw a near circuit diagram as shown in Figure 2.
2. Clean the end of connecting wires and arrange the apparatus as shown in Fig.2 where galvanometer G key K high resistance box (R.B) and cell are connected in series box (F.R.B) is connected across the galvanometer through the key k₂
3. Note the zero reading of a galvanometer and apply a high resistance R by the resistance box (R.B).
4. Close the key K₁ . Keeping the key open. Adjust R to get a large deflection. Note the deflection and find the corrected deflection by applying in zero error. The deflection should be equal to an even number of division.
5. Keeping R unchanged close both the keys and adjust the shunt resistance so that the deflection is reduced to half of its original value. The resistance G of the Galvanometer is equal to the shunt resistance S.

Repeat the experiment thrice for different values of R.

OBSERVATION & CALCULATION

No. of obs	Resistance R (ohms))	Deflection of Galvanometer		Half Deflection (div)	Shunt resistance S (ohm)	Resistance of galvanometer G = (ohm)
		Observed (div)	Corrected D (div)
1. 2. 3.

Mean resistance of the galvanometer

= G = _________ ohms.

Precautions

1. All the connections should be clean and tight.
2. High resistance in series with the galvanometer should be such as to get a large deflection preferably equal to even number of division.
3. While adjusting the shunt resistance the series resistance should not be changed.
4. Zero error of the galvanometer should either be removed or accounted for.
5. Key K₁ should be closed only after applying a large resistance R.

VIVA VOCE

Q.1      what is galvanometer?

Ans.    It is an electrical instrument used for the detection or measurement of very small values of current.

Q.2.     what is the difference between a galvanometer and an ammeter?

Ans.    An ammeter is simply a galvanometer with a suitable resistance connected I parallel with it. It is always connected in series with the circuit.

Q.3      why should the resistance R be very large in this experiment?

Ans.    A very large resistance R as compared with G or S is used so that the approximation used in deriving the final equation ca be justified circuit may not change appreciably by shunting the galvanometer.

Q.4.     Define Ohm’s law.

Ans.    The potential difference across a conductor is proportional to the current flowing through it provided its physical state remains the same.

Q.5.     what is galvanometer?

Ans.    It is instruments used detection and measurement of very small values of current.

Q.6.     what is the working principle of galvanometer?

Ans.    The galvanometer works on the principle that when a current  oils placed in magnetic field it experiences a torque which rotates it

Q.7.     what is shunt?

Ans.    It is a small resistance which is connected parallel to the coil of the galvanometer            .

Q.8.     What is the advantage of shunt?

Ans.    The advantage of the shunt is that most of current flows through it and so that instrument is protected   from damage due to heavy current flowing through the instrument.

Q.9.     Why the galvanometer show the zero deflection when the second key is closed?

Ans.    When the second key is closed then as the current always follows easy path, so almost at all the current goes to the zero resistance path that is why galvanometer shows the zero deflection.

Q.10.   Why the galvanometer gives half deflection when both keys are closed?

Ans.    Because half of the current passes through the shunt resistance ‘S’ and half through the galvanometer that is why this method is called half deflection method.

Q.11.   The resistance of which part of galvanometer is measured?

Ans.    The resistance of coil of galvanometer is measured.

Q.12    Write the formula for calculating the resistance of galvanometer by half deflection method?

Ans.    The formula is R_g = .

Q.13.   Under what condition the formula R_g =   reduce R_g =S?

Ans.    If S << R then R-S = R_g, hence we can write R_g =S.

Q.14.   What do you mean by sensitive galvanometer?

Ans.    A galvanometer which gives large deflection for a given small current is known as a sensitive galvanometer.

Q.15.   What is meant by dead beat galvanometer?

Ans.    A galvanometer, in which the coil comes to ret quickly after current passed through it, is called stable or a dead beat galvanometer.

Q.16.   A galvanometer is not shunted when its resistance is measured. Why?

Ans.    A galvanometer is not shunted when its resistance is measured because when it is shunted, this resistance decreases and its actual value will not be measured.