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  • A multirange current meter can be constructed by using a galvanometer circuit as shown in figure. We want a current meter that can measure 10 mA, 100 mA and 1 mA using a galvanometer of resistance 10

A multirange current meter can be constructed by using a galvanometer circuit as shown in figure. We want a current meter that can measure 10 mA, 100 mA and 1 mA using a galvanometer of resistance 10 \Omegaand that produces maximum deflection for current of 1 mA. Find S_{1}. S_{2}and S_{3} that have to be used.

Answers (1)

A galvanometer is sometimes also used as an ammeter by simply attaching a very low resistance wire also called as shunt S in parallel to the galvanometer. IgG = (I – Ig) S gives the relationship, in which range of galvanometer is Ig and R is the resistance of the galvanometer.

For measuring I _{1}=10mA: I_{G}.G=(I_{1}-I_{G})(S_{1}+S_{2}+S_{3})

For measuring I _{2}=100mA: I_{G}.(G+S_{1})=(I_{2}-I_{G})(S_{2}-S_{3})

For measuring I _{3}=1A: I_{G}.(G+S_{1}+S_{2})=(I_{3}-I_{G})(S_{3})

gives S_{1}= 1\Omega , S_{2}= 0.1 \Omega , S_{3}= 0.01 \Omega

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