Answer the follow

e/m or the specific charge is a important ratio as it is found in basic equations of electro-magnetism, for instance,

evB = mv²/r …(1)

→ (e/m)vB = v²/r

v = Br(e/m)

eV = 1/2mv² …(2)

→ v =

We see that the ratio (e/m) has an impact on the dynamics of electron in magnetic field hence e/m is given more importance that e or m.