First prototype ammonia maser and inventor Charles H. Townes. The ammonia nozzle is at left in the box, the four brass rods at center is the quadrupole state selector, and the resonant cavity is at right. The 24 GHz microwaves exit through the vertical waveguide Townes is adjusting. At bottom are the vacuum pumps.
In 1953 Charles H. Townes, while professor at Columbia University, invented the MASER (Microwave Amplification by Stimulated Emission of Radiation). It was a precursor to the LASER that amplifies light.