what is galvanometer ? why it can't be used to measure current in circuit ?how it can be converted to voltmeter ? how we can incres current sensitivity of galvanometer ?

An instrument used to detect, measure, and determine the direction of small electric currents by means of mechanical effects produced by a current-carrying coil in a magnetic field.

Since Galvanometer is a very sensitive instrument therefore it can’t measure heavy currents. In order to     convert a Galvanometer into an Ammeter, a very low resistance known as "shunt" resistance is     connected in parallel to Galvanometer. Value of shunt is so adjusted that most of the current passes     through the shunt. In this way a Galvanometer is converted into Ammeter and can measure heavy     currents without fully deflected.