"High Frequency Rays of Cosmic Origin, II: Mountain Peak and Airplane Observations", in Physical Review. **Determining that the Origin of Cosmic Rays is from Outside the Solar System (1926)**

**Determining that the Origin of Cosmic Rays is from Outside the Solar System (1926)** R.A. Millikan and R.M. Otis. "High Frequency Rays of Cosmic Origin, II: Mountain Peak and Airplane Observations", in Physical Review, June, 1926, volume 27 #6, pp 645-659 in the issue of pp 647-814. Tis is the full issue for June, extracted from a larger bound voluume. "During the 1890s the theory that electricity was conveyed by a miniscule unit, the electron, gained acceptance. In 1910 Robert Millikan (Nobel Prize in physics, 1923) succeeded in precisely determining the magnitude of the electron’s charge. Small electrically charged drops of oil were suspended between two metal plates where they were subjected to the downward force of gravity and the upward attraction of an electrical field. By measuring how the various drops of oil moved about, Millikan showed that their charge always was a multiple of a precisely determined charge—the electron’s charge."--Nobel Prize website 714.3.