(a) Explain, giving reasons, the basic difference in converting a galvanometer into (i) a voltmeter and (ii) an ammeter.

(b) Two long straight parallel conductors carrying steady current I₁ and I₂ are separated by a distance ‘d’. Explain briefly, with the help of a suitable diagram, how the magnetic field due to one conductor acts on the other. Hence deduce the expression for the force acting between the conductors. Mention the nature of this force. 

a)    
Conversion of galvanometer into ammeter:

A galvanometer may be converted into ammeter by using very small resistance in parallel with the galvanometer coil.
The small resistance connected in parallel is called a shunt. If G is resistance of galvanometer, I_g is current in galvanometer for full-scale deflection, then for conversion of galvanometer into ammeter of range I ampere, the shunt is given by,
 

Conversion of galvanometer into voltmeter,

A galvanometer may be converted into voltmeter by connecting high resistance (R) in series with the coil of galvanometer.
If V volt is the range of voltmeter formed, then series resistance becomes,

b)
Magnetic field produced by current I₁ at any point on conductor Rs is given by, 

 

Force acting on length l of the conductor RS will be,

An equal force is exerted into the wire PQ by the field of conductor RS which is given by, 

Thus, the force is attractive when the current is acting along the same direction.
When, current flows in opposite direction, the forces between the two conductors are repulsive.