Step 1: An ammeter is designed to measure larger currents with lower resistance, whereas a milli-ammeter measures smaller currents and therefore requires higher sensitivity.
Step 2: Since a milli-ammeter is more sensitive, it has higher internal resistance compared to an ammeter. \[ \therefore x_1<x_2 \] \[ \boxed{x_1<x_2} \]


In the given circuit the sliding contact is pulled outwards such that the electric current in the circuit changes at the rate of 8 A/s. At an instant when R is 12 Ω, the value of the current in the circuit will be A.