Answers to Question and Problems

1. Describe the type of meter movement used in analog meters.

The type of meter movement is coil. As current flows in the

movable coil which were supported by jeweled bearings between

the poles of a permanent magnet, then the core will be

magnetized. The coil experiences torque and rotates to align its

plane perpendicular to the line joining the poles of the permanent

magnet. The pointer deflects according to the change in the

direction of current flow.

2. What device is used to extend the range of a DC ammeter and

how is it connected to the basic meter?

A shunt resistor is used to extend the range of a DC

ammeter. It allows the measurement of too large current values

since it is placed in series with the load so that all of the current

to be measured will flow through it. It is connected across the

meter terminals and may be located inside (internal shunts) or

outside (external shunts) the meter case.

3. What device is used to extend the range of a DC voltmeter and

how is it connected to the basic meter?

A device called multiplier resistor is also added on the

circuit to extend the range of dc voltmeter. It will then affect the

voltage in the voltmeter but not the current. Adding a resistor

with the same resistance with the voltmeter, the range will be

doubled.

4. Why is it necessary that an ammeter be a low resistance

instrument? Why must a voltmeter be a high resistance

instrument?

Ammeters are used in series while voltmeters are used in

parallel between two points, where the measurement is to be

made. Ammeter is necessary to be a low resistance instrument

and the voltmeter must be a high resistance instrument so that

these meters would not alter the behavior of the circuit.

5. What type of basic meter is configured as a wattmeter?

Electrodynamometer meter is configured as a wattmeter.

An electrodynamometer is an instrument used for measuring the

electric power.

6. Two 150V voltmeters are being compared; meter A has 5kΩ/V

while meter B has a total meter resistance of 750kΩ. Which is the

more sensitive meter? Why?

Voltmeter sensitivity is expressed in ohms per volt. It is the

resistance of the voltmeter at the full-scale reading in volts. Using

the equation for computing voltmeter sensitivity which is total

resistance over the input resistance, meter B is more sensitive

meter who has 5KΩ/V than meter A which has 33.33 Ω/V.

7. What current is required for full scale deflection of a

galvanometer having a current sensitivity of 50μA per scale

division? The meter has exactly 50 divisions on either side of the

mid-scale index.

I=50 μAdivision

×2∗50divisions

I=5mA

8. What potential difference appears across the galvanometer

described in problem 6 when the pointer is fully deflected? The

meter resistance is 10Ω.

voltmeter sensitivity=5k Ω /V

Im=(5 k ΩV )−1

=2×10−4 A

V=Im Rm= (2×10−4 A )(10Ω)

V=2mV

9. Determine the series resistance needed to convert a

galvanometer to a voltmeter reading 15V on full scale deflection

if a current of 2.5mA causes full scale deflection and the

resistance of the coil is 10Ω.

V T=Im (R s+Rm )

R s=V T

Im−Rm=

15V0.0025 A

−10Ω

R s=5990Ω

10. Determine the shunt resistance needed to convert a

galvanometer to an ammeter with a full scale reading of 10A if

the scale requires 2.5mA to ca use full deflection with a coil

resistance of 10Ω.

R sh=ImRmI T−Im

=(0.0025 A )(10Ω)10 A−0.0025 A

R sh=2.501mΩ