# SSC JE Measurement & Instrumentation Solved Questions (2018-2009) Part-2

Ques 1. When a source is delivering maximum power to the load the efficiency will be? (SSC-JE 2015)

1. Below 50 %
2. Above 50%
3. 50 %
4. Maximum

Explanation:-

## Maximum Power Transfer theorem The maximum power is transferred when the load resistance RL is equal to internal resistance Rth or the equivalent resistance Rth.

RL = Rth Under this condition, the same amount of power is dissipated in the internal resistance Rth and hence the efficiency is 50%.

Ques 2. In the Maxwell Bridge as shown in the figure below the value of resistance Rx and inductance Lx of a coil is to be calculated after balancing the bridge. The component value is shown in the figure at balance the value of Rx and Lx will respectively be (SSC-JE 2015) 1. 37.5 ohms, 75 mH
2. 75 ohm, 75 mH
3. 375 ohm, 75 mH
4. 75 ohm, 150 mH

Explanation:-

A Maxwell bridge is a modification to a Wheatstone bridge used to measure an unknown inductance (usually of low Q value) in terms of calibrated resistance and inductance or resistance and capacitance.

The balanced condition for Maxwell bridge is given as

Z1Z4 = Z2Z3

Here Z1 = Rx + jωLx

Z2 = 200Ω

Z3 = 750Ω

Z4 = R4 ||(1/ωC4) Ques 3. A 150-volt moving iron voltmeter of accuracy class 1.0 reads 75 V when used in a circuit under standard conditions. The maximum possible percentage error in the reading is (SSC-JE 2015)

1. 2
2. 0.5
3. 4
4. 1

Explanation:-

percent error = [experimental value – theoretical value] / theoretical value x 100%

75 -150 = -75

|-75| = 75

%error = 75/150

= 0.5%

Ques 4. Creeping in a single-phase induction type energy meter may be due to (SSC-JE 2015)

1. Over Voltage
2. Vibrations
3. Overcompensation for Friction
4. All of the above

Explanation:-

The slow but continuous rotation of energy meter when only pressure coil is excited and there is no current flowing through the current coil (i.e under no-load condition) is known as creeping.

Creeping occurs due to

1. Overcompensation for friction.
2. The supply voltage is more than normal voltage
3. Stray magnetic field
4. Vibration

Ways to prevent creeping :

In order to prevent creeping two holes of the same radius is drilled on the opposite side of the spindle, the disc will stop when one of the holes comes under the shunt magnet.

Ques 5. With the positive probe on an NPN base, an ohmmeter reading between the other transistor terminal should be? (SSC-JE 2015)

1. Infinite
2. Open
3. Low resistance
4. High Resistance

Explanation:-

Since the base is connected to +ve probe it has a higher potential than other terminals so it makes the transistor in forward bias mode for both terminals and as we know in forward bias resistance is very low.

Ques 6. If a dynamometer type wattmeter is connected in an AC circuit, the power indicated by the wattmeter will be? (SSC-JE 2015)

1. Instantaneous Power
2. Peak Power
3. Volt-ampere product
4. Average power

Explanation:-

Wattmeter measure the active power P in the circuit. In the AC circuit, the active power at any instant is given by

P = VI

And the average power is given by which can be measured by wattmeter only is given as

P = VICosΦ

Where V and I are RMS value of voltage and current.

Ques 7.  A Lissajous pattern on an oscilloscope has 5 horizontal tangencies and 2 vertical tangencies. The frequency of the horizontal input is 1000 Hz. The frequency of the vertical input will be (SSC-JE 2015)

1. 400 Hz
2. 2500 Hz
3. 4000 Hz
4. 5000 Hz

Explanation:-

Lissajous Pattern with different Ratios is given as

Fy/fx =hx/hy

Fy: unknown frequency

Fx: known frequency

hy: tangencies at the y-axis

hx: tangencies at the x-axis

Fy/1000 = 5/2

Fy = 2500 Hz.

Ques 8. Three wattmeter method of power measurement can be used to measure power in (SSC-JE 2015)

1. Balanced Circuits
2. Unbalanced circuits
3. Both Balanced and Unbalanced circuits
4. None of the above Answer.3. Both Balanced and Unbalanced circuits

Explanation:-

In case if the supply is 3-phase, 4 wire system, then for the unbalanced system or loads the two wattmeters can’t be used since the current is flowing through neutral. The three wattmeter method of power measurement will work regardless of whether the load is balanced or unbalanced, delta or Wyne connected.

Ques 9. Modern electronic multimeter measure resistance by? (SSC-JE 2015)

1. Using the bridge circuit
2. Forcing a constant current and measuring the voltage across the unknown resistance
3. Using an electrical bridge circuit
4. Using an electronic bridge compensator for melling

Answer.2. Forcing a constant current and measuring the voltage across the unknown resistance

Explanation:-

The only type of meter used in modern electronics instruments is the moving coil or d’Arsonval. These equipment are accurate and Robust and may be obtained with extremely sensitive movements.

To measure resistance a known EMF is applied in series with the meter and the resistor, and Ohm’s law is used to perform the calibration. A protective resistor is placed in series with the meter, so that if the unknown resistance should, in fact, be zero, the meter will read the only full scale, instead of passing a destructively large current. The value of the protective resistor is subtracted in each case from the total calculated resistance to obtain the unknown resistance.

Ques 10. A CRO Screen has 10 divisions on the horizontal scale. If a voltage signal 5 sin (314t + 45º) is examined with a line base setting of 5m sec/div, the number of cycle of the signal displayed on the screen will be? (SSC-JE 2015)

1. 0.5 cycle
2. 2.5 cycle
3. 5 cycle
4. 10 cycles

Explanation:-

Voltage V = 5sin(314t + 45º)

Angular frequency (ω) = 314

ω = 2π/T

T = 2π/ω = 2π/314 = 0.02 msec

X scale of CRO = 5msec/division

Total X scale = 5/10 msec = 0.5 msec

Then the number if cycle of the waveform of the screen = total X scale/time

= 0.5/0.2 = 2.5 cycle.

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