Every bend is F = qv × B integrated live — the circle emerges, it is not drawn.
qV = ½mv² — the gun sets the speed honestly. Radius follows √V, never V.
Stronger coils, tighter circles: r = √(2Vm/e) ÷ B. The period chip won't move with the gun — T = 2πm/(eB) is speed-free.
0° draws a circle; tilted draws a helix — v⊥ sets the radius, v∥ sets the pitch.
Crossed E × B: the beam flies straight to the flag only when v = E/B. Tune it, then read the hit. With plates on, the circle fit sits out — crossed fields bend it.
Selector tune needs ≈703 V — beyond the 600 V plate range at this gun voltage and field, so the beam keeps gyrating. Lower the accelerating voltage (or the coil field) to bring v = E/B within reach.
Beam off. Start it and the Lorentz force does the rest — the circle, the helix and the selector line all emerge from F = q(E + v × B).