Definition of Magnetic Field
Magnetic field B is defined operationally by the magnetic force on a test charge.
(We did this to talk about the electric field too)
What is "magnetic force"? How is it distinguished from "electric" force?
Start with some observations: CRT deflection
http://wug.physics.uiuc.edu/courses/phys112/fall97/acts/lect12/act12a/sld001.htm
- Empirical facts: a) magnitude: µ to velocity of q
b) direction: ^ to direction of q’s velocity
c) direction: ^ to direction of B