Problem 3.206

$3.206 .$ A coil of radius $r=25 \mathrm{~cm}$ wound of a thin copper wire of length $l=500 \mathrm{~m}$ rotates with an angular velocity $\omega=300$ rad/s about its axis. The coil is connected to a ballistic galvanometer by means of sliding contacts. The total resistance of the circuit is equal to $R=21 \Omega$. Find the specific charge of current carriers in copper if a sudden stoppage of the coil makes a charge $q=10^{\circ} \mathrm{nC}$ flow through the galvanometer.