A galvanometer has a coil of resistance `100(Omega)`showing a full-scale deflection at 50(mu)A. What resistance should be added to use it as (a) a voltmeter of range 50 V (b) an ammeter of range 10 mA?

Explanation

(a) When a potential difference of 50 V is applied across the voltmeter, full-scale deflection should take place. Thus,`50(mu)A `should go through the coil. We add a resistance R in series with the given coil to achieve this
we have,
`50(mu)A=(50V)/(100 Omega+R)`
`R=10^(6)(omega)-100(Omega)~~10^6(Omega)`.
(b) When a current of 10mA is passed through the ammeter, `50(mu)A should go through the coil. We add a resistance r in parallel to the coil to achieve this ,
The current through the coil is
`50(mu)A=(10 mA)(r)/(r+100 Omega)`
`or, r~~0.5(Omega).`