How it began
Ernst Ruska and Max Knoll, working in Berlin, built the first electron microscope in 1931. The work grew out of Ruska's thesis under Knoll's guidance, an effort to build a microscope that used electrons instead of light waves. It followed Hans Busch's invention of the first electromagnetic lens in 1926. By 1933 Ruska had built a new instrument with dual magnetic coils as lenses, by analogy with a compound optical microscope, which gave resolution beyond that of the light microscope.
Before it existed
Light microscopes were approaching the theoretical limit of their resolution, which is set by the wavelength of light. George Airy had theorized such a limit in 1834, and Ernst Abbe and Hermann von Helmholtz reached the same conclusion in 1873. Microscopes using ultraviolet light improved resolution only modestly, and X-ray microscopy was hard to realize. In 1924 Louis de Broglie's hypothesis pointed to electrons as a possible new medium.
How it works
The instrument replaces light rays and glass lenses with an electron beam and electron lenses, which are magnetic coils. Because the wavelength of electrons is smaller than that of light by a factor of about 100,000, far finer detail can be resolved. The first model had very modest magnification, but Ruska's 1933 microscope reached around 12,000 times.
How it spread
By 1938 Ruska had produced a prototype with a magnification of 30,000, and Siemens released the first commercial electron microscope that year. From then on, transmission electron microscopes became more readily available in other parts of the world, including North America. In 1937 Manfred von Ardenne developed the first scanning electron microscope.
Who pushed back
Commercial developers were at first unimpressed and questioned who would use the instrument. Biologists were the main target, but the electron beam heated up and destroyed their samples. Ruska's brother Helmut, a young medical doctor, persuaded his former professor Richard Siebeck to write a favorable assessment, which convinced Siemens and Carl Zeiss to fund commercial development.
Where it stands today
Ernst Ruska received the Nobel Prize in Physics in 1986 for the invention, sharing it with Gerd Binnig and Heinrich Rohrer, who developed the scanning tunneling microscope. Current electron microscopes reach atomic-level resolution, helped by computer technology that made accurate focusing possible.
