ELECTRICAL MEASURING INSTRUMENTS

Chapter - 15

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Q.1: What is the function of the concave pole pieces and the coaxial soft iron cylinder in the moving coil galvanometer?
Ans: The concave magnetic poles and the cylindrical core make the magnetic field radial and stronger (so the current becomes directly proportional to the deflection).

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Q.2: Why is it necessary to have some form of controlling couple in the moving coil galvanometer?
Ans: Controlling couple is necessary to control the motion of the coil, it is proportional to the current to be measured. It is produced by using a spring control method, which consists of two hair springs attached to a spindle wound in the opposite directions. As the coil rotates the spring winds up and produces a counter torque. The coil comes to rest (the final deflection of the pointer is given) when the deflecting torque (or magnetic torque) is counterbalanced by the controlling torque (or restoring torque).

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Q.3: What is meant by the sensitivity of a galvanometer? On what a factor does it depend? How can we have large sensitivity of a moving coil galvanometer?
Ans: A galvanometer is sensitive if it gives large deflection for a very small current. The sensitivity of a galvanometer is the current in microamperes required to cause a deflection of 1mm or 1 division.

$$S = \frac{I (\text{in } \mu A)}{\theta (\text{in div})}$$

Since $I = K \cdot \theta$, hence the galvanometer is sensitive if $K (= C/BAN)$ is small. Sensitivity depends on $C$ (couple per unit twist), $N$ (number of turns), $A$ (area of coil) and $B$ (strength of magnetic field).

For large sensitivity a soft iron core (sphere or cylinder) is placed inside the coil and the poles are made circular or cylindrical. This makes $4B$ stronger and radial.

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Q.4: Which galvanometer usually has greater sensitivity, aluminum pointer or lamp and scale type? Why?
Ans: Lamp and scale type galvanometer has greater sensitivity ($10^{-15}$ A/div), because it gives large deflection for a very small current.

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Q.5: We want to convert a galvanometer into (a) an ammeter (b) a voltmeter. What do we need to do in each case?
Ans:

• To convert a galvanometer into an ammeter, we connect a low resistance in parallel (called shunt).
• To convert a galvanometer into a voltmeter, we connect a high resistance in a series (called multiplier).

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Q.6: Why is it necessary for an ammeter to have zero or negligibly small resistance?
Ans: An ammeter must have negligibly small resistance so that it may not alter the current being measured.

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Q.7: What necessary condition must a voltage measuring device satisfy?
Ans: A voltage measuring device must contain a very high (in fact, infinite) resistance, so that it will, practically, draw no current from the circuit across which it is connected.

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Q.8: Why must an ammeter be connected to a circuit in series and a voltmeter in parallel?
Ans: An ammeter must be connected in series to a circuit because its resistance is very small as compared to the total resistance of the circuit. Hence it does not alter the current being measured. But a voltmeter has very high resistance, so it must be connected in parallel to a circuit.

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Q.9: An ammeter and voltmeter of suitable ranges are to be used in a circuit. What might happen if by their mistake positions are interchanged?
Ans:

• If, by a mistake, an ammeter is connected in parallel to a circuit, its coil will be burnt out to heavy current (because of its extremely low resistance).
• When a voltmeter, by mistake, is connected in series to a circuit it will give reading but will not record correct p.d. because it will decrease the current (due to its very high resistance). It will not cause damage.

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Q.10: The terminals of ammeters are usually made of thick and bare metal while those of voltmeters are quite thin and well insulated. Explain why?
Ans:

• An ammeter must have very low resistance. So its terminals should have almost zero resistance. Hence terminals must be made of thick, bare metal.
• A voltmeter must have very high resistance. So its terminals should be thin and well insulated to avoid sparking between the terminals.

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Q.11: Why is a potentiometer considered one of the most accurate voltage measuring devices?
Ans: The principle of a potentiometer is that the potential drop across any length of wire of uniform cross-section is directly proportional to the length of the wire. At the balance point, the two terminals of the galvanometer are at the same potential, and no current will flow through it. Hence, a potentiometer is an instrument that can be used to measure the emf of a source and compare potentials without drawing any current from the source. Essentially, it balances an unknown p.d. against an adjustable, measurable p.d.

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Q.12: How is a Wheatstone bridge used for measuring an unknown resistance?
Ans: If we connect three resistances $R_1$, $R_2$, and $R_3$ of precisely known adjustable values and a fourth unknown resistance $R_4$, and these are so adjusted that the galvanometer shows no deflection; then in this balanced condition, $R$ square root $R_2$ is equal to $R_3$ square root $R_4$. Hence $R_4$ can be calculated.