what is the idea in using Helmholtz coil for detecting the ratio of charge to mass? e/m?

Answer 1

putting a charged particle through a known magnetic field at a known speed, and measuing the deflection of the particle will determine the mass-charge ratio, since the deflection is dependant on the charge and the mass and from it the ratio can be determined...
(more charge is more force, more mass means less deflection per unit force)

i think the Helmholtz coil is used as it provides a fairly uniform magnetic field

http://en.wikipedia.org/wiki/Mass_spectrometer

Answer 2

So that's why you wanted to know what e/m was!