In 1931, Max Knoll and Ernst Ruska generated the first magnified images of mesh grids placed over an anode aperture. This device used two magnetic lenses to achieve higher magnifications than any optical microscope could manage at that time. The team worked under Adolf Matthias at the Technische Hochschule in Charlottenburg, now known as the Technische Universität Berlin. Earlier experiments by Hertz in 1883 had demonstrated manipulation of electron beam direction using electrostatic and magnetic deflection. Emil Wiechert focused electrons with an axial magnetic field in 1899, while Arthur Wehnelt improved oxide-coated cathodes in 1905. Hans Busch developed the electromagnetic lens in 1926, providing a critical component for later instruments. Leó Szilárd tried to convince Dennis Gabor to build an electron microscope in 1928 after filing his own patent. Siemens engineer Reinhold Rüdenberg filed U.S. Patent No. 2058914 and 2070318 in 1932, claiming independent invention of the electron microscope. He stated Siemens had been working on this technology for years before those patents were filed. Ruska and Knoll built their first functional instrument in 1933 that exceeded the resolution of light microscopes. Siemens financed Ernst Ruska and Bodo von Borries in 1937 to develop applications for biological specimens. Manfred von Ardenne pioneered the scanning electron microscope in 1937. The first commercial electron microscope appeared from Siemens in 1938. Max Knoll died in 1969 and did not receive a share of the 1986 Nobel prize. Reinhold Rüdenberg also died in 1961 without receiving recognition for the invention.
Transmission Microscope Mechanics
A transmission electron microscope uses a high voltage electron beam to illuminate thin specimens and create images. Electrons typically have energies ranging from 20 to 400 keV when produced by an electron gun. These electrons are focused by electromagnetic lenses and transmitted through a sample thinner than most human hair. When the beam emerges from the specimen, it carries information about the internal structure of the material. This spatial variation forms what operators call the image. Early viewing methods involved projecting magnified electron images onto fluorescent screens coated with zinc sulfide or other phosphor materials. Modern systems often couple high-resolution phosphors via fiber optic light-guides to digital camera sensors. Direct electron detectors now exist that eliminate scintillators entirely to address limitations of older cameras. For many years, resolution was limited primarily by spherical aberration in the electron optics. Hardware correctors introduced around the start of the 21st century reduced these aberrations significantly. High-resolution transmission electron microscopy can now achieve resolutions below 0.5 angstroms, which equals 50 picometers. This capability enables magnifications exceeding 50 million times while determining atomic positions within materials. Harald Rose and Maximilian Haider demonstrated aberration correction in TEM mode in 1998 using a hexapole corrector. Ondrej Krivanek and Niklas Dellby achieved similar results in STEM mode in 1999 with quadrupole/octupole correctors.