When was the electronvolt first introduced?

Question

When was the unit "electronvolt" (eV) first introduced, and by whom?

The definition of the eV requires the knowledge of the electron charge $e$, so it must have been introduced after 1909, the date of the famous oil drop experiment conducted by R. A. Millikan and H. Fletcher, in which, as far as I know, $e$ was measured for the first time.

Answer 1

The OED's "electronvolt" entry gives this as the earliest usage in English:

• 1925 Proc. Royal Soc. A. 109 559

  Now one electron-volt = 1·59 .10⁻¹² ergs, and 4·18 .10⁻¹⁷ ergs = 1 gram-calorie.

But it defines what its measure is, not what it is.