Significance
In some experiments, the charge-to-mass ratio is the only quantity that can be measured directly. Often, the charge can be inferred from theoretical considerations, so that the charge-to-mass ratio provides a way to calculate the mass of a particle.
Often, the charge-to-mass ratio can be determined from observing the deflection of a charged particle in an external magnetic field. The cyclotron equation, combined with other information such as the kinetic energy of the particle, will give the charge-to-mass ratio. One application of this principle is the mass spectrometer. The same principle can be used to extract information in experiments involving the Wilson cloud chamber.
The ratio of electrostatic to gravitational forces between two particles will be proportional to the product of their charge-to-mass ratios.
wikipedia
also, did Thompson turn off the magnetic field or the electric field after balancing them?
after a bit of googling I'm getting alot of different answers :/