question_answer

Directions (Q. No. 6 to 10):

Read the following text and answer the following questions on the basis of the same:

Shunt resistance:

The ammeter shunt is the device which provides the low resistance path to the flow of current. It is connected in parallel with the ammeter. In some ammeter the shunt is in-built inside the instrument while in others it is externally connected to the circuit Ammeters are designed for measurement of low current. For measuring high current, the shunt is connected in parallel to the ammeter. The significant portion of the current passes to the shunt because of the low resistance path and little amount of current passes through the ammeter.

The shunt is connected in parallel to the ammeter because of which the voltage drops across the meter and shunt remain the same. Thus, the movement of the pointer is not affected by the shunt.

Let us consider that the current to be measured is

I. The circuit has ammeter and shunt connected parallel to each other. The ammeter is designed for measurement of small current say, \[{{\operatorname{I}}_{m}}\]. The magnitude of the current I passes through the meter is very high, and it will burn the meter. So, for measuring the current I the shunt is required in the circuit.

As the shunt connects in parallel with the ammeter, thus the same voltage drops occur between them:

Shunt current \[{{\operatorname{I}}_{SH}}=I-{{I}_{m}}\]

So,     \[{{\operatorname{R}}_{SH}}={{I}_{m}}{{R}_{m}}/(I-{{\operatorname{I}}_{m}})\]

\[\therefore \ \ \ \ \ \ \ \operatorname{I}/{{I}_{m}}\ =1+({{R}_{m}}/{{R}_{SH}})\]

The ratio of the total current to the current required for the movement of the ammeter coil is called the multiplying power of the shunt.

\[\therefore \]The multiplying power = m = \[\operatorname{I}/{{I}_{m}}\]

\[{{R}_{SH}}={{R}_{m}}/(m-1)\]

The following are the requirements of the shunt.

• The resistance of the shunt should remain constant with time.

• The temperature of the material should remain same even though substantial current flows through the circuit.

Manganin and Constantan are used for making the shunt of DC and AC ammeter respectively. What is multiplying power of the shunt?

A) \[\frac{Total current}{Current required by the movement\ of ammeter coil}\]

B) \[\frac{\begin{array}{*{35}{l}}    Current required by the movement  \\    \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ of ammeter coi  \\ \end{array}}{Total current}\]

C) Current required by the movement of ammeter coil X Total current

D) None of the above