Proton charge-to-mass quotient

This is the number that determines how a proton responds to electromagnetic fields, and it is 1836 times smaller than the electron's, so protons are correspondingly harder to steer. A proton's cyclotron frequency is 15.2 MHz per tesla against the electron's 28 GHz per tesla: the reason cyclotrons and synchrotrons are big and heavy while an electron gun fits in a desk lamp.

Mass spectrometry works entirely in this currency: an instrument separates ions by m/q, so a singly charged ion of mass number A appears at A while a doubly charged one appears at A/2, and resolving power is quoted against the proton's ratio. Penning-trap comparisons of proton and electron cyclotron frequencies in the same field are also how the proton-electron mass ratio is measured to parts in 10¹¹, since the field cancels out of the comparison.