A charged particle of charge e and mass m is moving in an electric field E and magnetic field B. Construct dimensionless quantities and quantities of dimension [T ]–1.
The quantities provided are charge, electric field and magnetic field. Using these three we can never eliminate the units to unity. Hence, there is no such constructible dimensionless quantity.
[T]-1 is the dimension of frequency.
The time period of a cyclotron is,
Hence the frequency is going to be,
This is the [T]-1 quantity.