First slide

Tangent Galvanometer

Question

An ammeter is obtained by shunting a 30 $Ω$ galvanometer with a 30 $Ω$ resistance. What additional shunt should be connected across it to double the range ?

Moderate

A

15 $Ω$
B

10 $Ω$
C

5 $Ω$
D

None of the above

Solution

For ammeter $S = (\frac{i_{g}}{i - i_{g}}) \times G$
$∴ \frac{S}{G} = \frac{i_{g}}{i - i_{g}} or \frac{G}{S} = (\frac{i}{i_{g}} - 1)$
or $\frac{30}{30} = \frac{i}{i_{g}} - 1 or i = 2 i_{g}$
New range is doubled i.e., 4i_g
Now shunt required
$S = (\frac{i_{g}}{4 i_{g} - i_{g}}) 30 = 10 Ω$
This can be obtained by shunting the earlier shunt of 30 $Ω$ ith a new shunt of 15 $Ω$

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