How did Charles H. Townes create the first working maser?

In 1951, American physicist Charles H. Townes conceived a method to generate stimulated emission at microwave frequencies. By the end of 1953, Townes and two students successfully demonstrated the first working device, which he named the maser—an acronym for "microwave amplification by stimulated emission of radiation."123

To create the device, Townes used a beam of ammonia molecules. The apparatus functioned by passing this beam along the axis of a cylindrical cage of metal rods with alternating positive and negative electric charges. This nonuniform electric field sorted the molecules, focusing those in an excited state through a small hole into a resonant microwave cavity. Within this cavity, the excited molecules were induced to emit a pure microwave frequency, effectively amplifying the radiation.13

This invention was a landmark achievement in physics, serving as the principal precursor to the laser. For his foundational role in the development of both the maser and the laser, Townes was awarded the Nobel Prize for Physics in 1964.123