A galvanometer with a coil of resistance $120\,\Omega$ shows full-scale deflection for a current of $2.5\,\text{mA}$. How will you convert the galvanometer into an ammeter of range $0$ to $7.5\,\text{A}$? Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Justify your answer.

Shunt $S\approx0.04\,\Omega$ in parallel; net resistance $\approx0.04\,\Omega$; it reads slightly less than the actual current.

CBSE Class 12 · 2005 · Magnetic Effects of CurrentShort Answer

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A galvanometer with a coil of resistance

120 Ω

shows full-scale deflection for a current of

2.5 mA

. How will you convert the galvanometer into an ammeter of range

0

7.5 A

? Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Justify your answer.

Solution

G = 120 Ω

I_{g} = 2.5 mA = 0.0025 A

I = 7.5 A

. Shunt required:

S = \frac{I _{g}}{I - I _{g}} G = \frac{0.0025}{7.5 - 0.0025} \times 120 \approx 0.04 Ω.

Connecting this

0.04 Ω

shunt in parallel gives an ammeter of range

0

–

7.5 A

. Net resistance

R_{A} = \frac{G S}{G + S} = \frac{120 \times 0.04}{120.04} \approx 0.04 Ω.

Because the ammeter has a small but non-zero resistance, inserting it in series slightly increases the circuit resistance and so it reads slightly less than the original current.

CBSE Class 12 Board · 2005 · Subjective

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CBSE Class 12 · 2005 · Magnetic Effects of CurrentShort Answer

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