E/M tube: Circular bending of an electron beam in a magnetic field.

E/M tube: Circular bending of an electron beam in a magnetic field. This apparatus is designed to measure e/m, the charge to mass ratio of the electron; similar to the method used by J.J. Thompson in 1897. A glass bulb is evacuated, except for a trace amount of helium. A beam of electrons is generated by a heater filament, then accelerated through a known potential V; so the velocity is known. When a current I flows in a pair of parallel Helmholtz coils, one on either side of the tube, a uniform magnetic field B is produced at right angles to the electron beam. This magnetic field deflects the beam in a circular path with radius r, which can be measured by a mirrored cm. scale. The beam is visible because the electrons collide with the helium atoms which are excited, then emit bluish light. The ratio e/m = 2V/B r . The coils have a radius and separation of 15 cm. Each coil has 130 turns. The diameter of the glass bulb is 13 cm. V is varied from 150 to 300 V.D.C.. Heater voltage is 6.3 V(AC orDC). B is the product of I times 7.80x10 tesla/amp. I should be kept smaller than 3 amps (at 6-9 V.D.C.). 2 -4 2 Notes: Needs very dark room. _Stand-by_ switch provides partial voltage to the filament, shortening warm-up time.