
$\frac{m v_{0}^{2}}{r}=q v_{0} B_{0} \Rightarrow r=\frac{m v_{0}}{q B_{0}}$
Also, from geometry, $L=r \sin 30^{\circ} \Rightarrow r=2 L$
or $L=\frac{r}{2}=\frac{m v_{0}}{2 q B_{0}}$
The magnitude of the magnetic field $(B)$ due to the loop $ABCD$ at the origin $(O)$ is :
Reason : Cyclotron frequency depends upon the velocity
Reason : Resistance of a voltmeter is very large.
