= Solution
A heated cathode emits <Electron>[electrons] into low-pressure atomic vapor, and a variable accelerating <voltage> gives them <kinetic energy>. A grid and small retarding region precede the collecting anode. Below the first excitation threshold most collisions are elastic; above it, <Electron>[electrons] can transfer a definite <excitation energy> to <atom (Physics)>[atoms] and may then fail to overcome the collector's retarding field. Collector current develops repeated minima as accelerating <voltage> increases, separated by roughly the same excitation <voltage>, approximately 4.9 V for mercury. Contact potentials and <energy> spreads shift or smear the minima without removing their characteristic spacing. A smooth loss mechanism would not produce these repeated fixed-energy thresholds.<Über Zusammenstöße zwischen Elektronen und den Molekülen des Quecksilberdampfes und die Ionisierungsspannung desselben by James Franck and Gustav Ludwig Hertz>[1914, accelerated-electron energy losses in mercury]{ref}
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