Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Shielding and Grounding of Bridges”.
1. In general bridges consist of __________
a) lumped inductances
b) lumped resistances
c) distributed capacitance
d) distributed impedance
Clarification: AC bridges in general such as Maxwell, Anderson, Schering, etc consist of lumped components such as inductance.
2. Stray capacitance exists between the components.
Clarification: In general in AC bridges, stray capacitance exists between the components of the bridge with respect to the ground. These stray capacitances short the ratio arms and lead to errors in the measurement.
3. The magnitude of stray capacitances is fixed.
Clarification: Stray capacitances are uncertain in nature. Their magnitude depends on the adjustment of the bridge arms and the position of the operator.
4. Stray capacitance effects can be minimized by __________
a) making use of an inductance
b) connecting a resistance in series
c) shielding and grounding
d) using a galvanometer
Clarification: The stray capacitance effects in an AC bridge can be eliminated by shielding and grounding. This method helps in making the stray capacitances constant in value. They can be compensated.
5. Most popular method of avoiding the stray capacitance effects is __________
a) by grounding
b) by using guard rings
c) by using galvanometer
d) by using Wagner device
Clarification: A Wagner earthing device is used in general to eliminate the stray capacitance effects in AC bridges. The stray capacitance effects between the components in the ratio arms with respect to ground can be eliminated through this method.
6. Wagner device is a ____________
a) capacitance bridge
b) resistance bridge
c) inductance bridge
d) impedance bridge
Clarification: A Wagner earth device is generally used for shielding and grounding purpose. It consists of capacitances in the ratio arms along with a series RC combination connected across the ends of the bridge forming a potential divider.
7. Which is the guarding arm?
a) parallel RC combination
b) series RC combination
c) resistance R
d) capacitance C
Clarification: The series combination of R and C in a Wagner earth device forms a potential divider across the ratio arms. It is also known as the guard arm.
8. Bridge is suitable for _________
a) coils with high Q values
b) coils with low Q values
c) coils with intermediate Q values
d) coils with very high Q values
Clarification: The Maxwell inductance capacitance bridge is basically used for the measurement of quality factor (Q) of inductance coils with low Q values.