Corpuscle

Corpuscle, a dark-haired experimenter, leans beside a glass tube carrying a straight green beam.

The Physics

In 1897 J. J. Thomson crossed electric and magnetic fields over a cathode-ray beam (“Cathode Rays”, Phil. Mag. 44, 293). He tuned them so the beam ran straight, which gives its speed, v = E/B; then the deflection from one field alone gives the mass-to-charge ratio. The result was about a thousand times smaller than for the hydrogen ion, whatever the gas or electrode metal. He called the particles corpuscles: we call them electrons.

v = E/B

• v: beam speed (m/s)
• E: electric field (V/m)
• B: magnetic field (T)
• m/e: mass-to-charge ratio of the particle (kg/C)

Thomson measured only the ratio. The charge itself came from Millikan's oil-drop work (1909–13); modern values give an electron about 1/1836 of the hydrogen ion's mass.

Honest caveat: His 1897 ratio was rough (about 1/1000 rather than 1/1836), and others including Wiechert and Kaufmann measured similar ratios that year.

On the sticker

Corpuscle sits beside a glass discharge tube with a straight glowing green beam. Printed text: CORPUSCLE with the line m/e << H ION. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.