This horn antenna was used by Harold Ewen and Edward Purcell at Harvard University to search for the 21 cm emission from neutral hydrogen in the Milky Way Galaxy. They finally detected it on March 25, 1951.
In 1950, Ewen was a physics graduate student building a receiver to detect the 21 cm line of neutral hydrogen. Purcell, his supervisor, asked for, and received, a grant of $500 from the Rumford Fund of the American Academy of Arts and Sciences for materials costs. Ewen installed the horn outside the fourth floor of the Lyman Lab at Harvard, with the waveguide leading in through the window to the receiver and recorder. In heavy rains, the horn antenna funneled water into the lab. During the winter, passing students found the horn a tempting target for snowballs.