English: The first maser (Microwave Amplification by Stimulated Emission of Radiation), invented by Charles H. Townes, James P. Gordon, and H. J. Zeiger at Columbia University in 1953. Townes, Nikolay Basov and Alexander Prokhorov were awarded the 1964 Nobel Prize in Physics for theoretical research that led to the maser. Townes is shown with the device. This was an ammonia maser. The brass box is the vacuum chamber through which the ammonia ions travel. The ammonia gas nozzle is at left. The four rods at center are the quadrupole filter which filters out the lower state ammonia molecules, leaving a population inversion. In the resonant cavity at right, stimulated emission of microwaves by the molecules excites standing waves in the cavity, which pass out through the vertical output waveguide. The devices at bottom are vacuum pumps which evacuated the box. Masers are used as the timekeeping elements in atomic clocks, and as extremely low-noise microwave amplifiers in radio telescopes. Alterations to image: cropped out rest of magazine cover