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Answers to Review Questions: Ch # 15 Electrical Measuring Instruments

XII BSEK ( Karachi Board)

Q.1 What is the function of concave pole pieces and the coaxial soft-iron cylinder in moving coil galvanometer?
Ans. The function of concave pole pieces is to make the magnetic field radial and it also makes the field stronger.

Q.2 Why is it necessary to have some form of controlling couple in Moving coil galvanometer?
Ans.Yes it is necessary to make the cylinder back to its original position that is why we use the spring connected to that soft-iron cylinder and also there is a reason that the controlling torque must be equal to the deflecting torque.

Q.3 What is meant by Sensitivity of galvanometer? Write Factors? How the sensitivity of galvanometer is increased?
Ans.A galvanometer is sensitive when it gives large deflection foe the small current. It depends on c(Couple per twist),N(number of turns),A(area) and B(magnetic field)
The sensitivity is increased by the use of concave pole pieces and soft-iron cylinder.

Q.4 Which galvanometer usually has grater sensitivity, Aluminium pointer scale type or lamp and scale type? Why?
Ans.Lamp and pointer type has the larger sensitivity nearly equal to 10-5 A/div (it means that it will produce 100000 div when current of 1 ampere is passes through it)

Q.6 Why is it necessary for an ammeter to have zero or negligibly small resistance?
Ans.Because if we use the larger resistance it can alter the current which is being measured so w have to use the resistance of zero ohms or a negligibly small resistance.

Q.7 What necessary condition must a voltage measuring device satisfy?
Ans.The voltage measuring device must have very large resistance so that it can not draw any current from the circuit to which it is connected parallel .It can be really helpful for the measurement of potential difference.

Q.8 Why must an ammeter be connected to a circuit in series and voltmeter in parallel?
Ans.As we know that the current remains same in series with the circuit so to measure it correctly we have to connect the ammeter in series while the Voltage remains same parallel with the circuit so to measure it correctly we have to connect the voltmeter parallel to it.

Q.9 An ammeter and a voltmeter of just suitable ranges are to be used by circuit. What might happen if their positions are interchanged?
Ans.There will be two different effects or that .First if the ammeter is connected parallel its coil will be burnt because it has small resistance and the current will be heavy.
While if the voltmeter is connected in series it will give the wrong reading because of its high  resistance it will alter the current.

Q.10 The terminals of ammeter are usually made of thick and bare wire while those of voltmeters are quite thin and well insulated. Explain Why?
Ans. For ammeter we need small resistance so its coil are usually thick and bare metal while to avoid sparking the voltmeter should have high resistance is its coil is very thin and insulated.

Q.11 Why is a potentiometer considered one of the most accurate voltage measuring device?
Ans.Because it can measure the potential difference accurately with out drawing any current from the circuit.

Q.12 Describe a circuit that gives a continuously varying potential difference b/w zero and a certain maximum value.
Ans.The arrangement as shown in the figure is known as Potential divider by using this arrangement we can vary the potential difference by moving the pointer.

Q14 In a balanced wheat stone bridge, will the balance be affected if the positions of the cell and the galvanometer are interchanged?
Ans.No effect because the principle of bridge is to connect the two ends with a battery and the remaining two with the galvanometer.

Q.15 In a slide wire bride, is it absolutely necessary to have the bridge wire one meter long?
Ans.No it can be of any size.

Q.16 A Post office box is compact wheat stone bridge. Then why is it so named?
Ans.Because it was first used for the fault finding purpose in the Post office and for telegraph problems.

Q.17 Which is the more accurate instrument a meter bridge or a P.O Box?
Ans.P.O Box is more accurate because the higher the ratio arm grater will be the accuracy.


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