The first electron microscope was built in 1931 by Max Knoll and Ernst Ruska. Built at the Technical University of Berlin, Germany, it consisted of two electromagnetic lenses in series, and had a magnification factor of 17. However, with improvements and modifications three years later Ruska achieved a magnification factor of 12,000. In 1936 the electron microscope became commercially available.
The operation is based largely on a discovery by another German, Hans Busch, that in the same way that a glass lens can focus a beam of light, so a magnetic coil can focus a beam of electrons moving through it. As the wavelength of the electron is much shorter than that of the shortest wavelength of light, the electron beam and magnetic coil can form images of objects far smaller than those visible with a conventional, or optical, microscope. The electron beam is created with a high voltage (over 20,000 volts) electron gun similar to that inside a television set. This is positioned at the top, the beam being focused by the magnetic coils and projected onto a fluorescent screen, again similar to a television set.
Modern versions now have a column of lenses beginning at the top with the condenser lenses, through an objector lens and finally a projector lens. Unlike the standard optical microscope, the eyepiece is at the bottom of the microscope. A typical magnification factor is around 200,000, although one million times magnification can be achieved. However, many samples deteriorate under the severe bombardment of electrons necessary to magnify an object by one million times.
There are now various types of electron microscope, common versions being the scanning electron microscope and the transmission electron microscope. There are also many applications, primarily in the fields of medical research.
