Physics
Charge-to-mass ratio
Accelerate electrons, bend them in Helmholtz coils, measure the circle: e/m = 2U/(B^2 r^2), with the field integrated from Biot-Savart.
What happens
Two things are computed rather than assumed. The field is integrated from Biot-Savart over both coils, so the uniformity of a Helmholtz pair is a result - it matches the closed form at the centre to a tenth of a percent and stops matching a few centimetres out. The orbit is pushed with a Boris scheme, which keeps the speed exact, and e/m is then read back from the radius the trace actually reached.
Equations used in the module
Equations e V = ½ m v², e v B = m v² / r e/m = 2 V / (B² r²) B from Biot–Savart, integrated over the coils
