John Cunningham McLennan (14.04.1867 — 9.10.1935) was a Canadian physicist and a Fellow of the Royal Society of Canada (1903), of which he was president in 1924. He was born in Ingersoll. He graduated from the university in Toronto (1892) and worked there until 1932 (from 1904 as director of the cryogenic laboratory and from 1907 as professor).
His work was in the field of low-temperature physics, cryogenic engineering, radioactivity, spectroscopy, the electrical conductivity of gases and nuclear physics. In 1923 he built a hydrogen and then a helium liquefier and began research in the region of helium temperatures. He observed the character of the boiling of liquid helium at reduced pressure and noted a change in its properties once the temperature fell below the temperature corresponding to the maximum density (2.2 K), but he gave no interpretation of the observed fact (what was in fact being observed were the two kinds of liquid helium, He I and He II). He initiated (1903) the study of highly penetrating radiation, showing that “atmospheric rays” are able to penetrate thick screens (whence the name penetrating radiation) and to affect the rate of discharge of an electroscope.
A Fellow of the Royal Society (1915).