The electron microscope requires that the electron beam be in a vacuum, because electrons cannot travel far in air at atmospheric pressure. Pumps empty the column and specimen chamber of the electron microscope. Living specimens cannot be examined with an electron microscope, since they will not survive in a vacuum.
The magnification in magnetic electron microscopes is determined by the strength of the current passing through the electric and electromagnetic lens coils. Changing the current through the objective lens coil focuses the image. In the optical microscope the image is determined by absorption of light by the specimen; in the electron microscope the image results from a scattering of electrons by atoms of the specimen. Since an atom, with a high atomic number, have more electrons than a light atom, it appears darker.
As the beam passes through a specimen, each tiny differences in the structure of the specimen causes a variation in the electron stream. The image produced is then projected onto a fluorescent screen or recorded on film. The electron microscope, with its tremendous resolving power, can magnify specimens over 50,000 times.