The illumination source (or electron gun) in a thermo-ionic emission TEM works much like a light bulb. A filament (cathode) is the source of electrons. It is usually a hairpin-shaped tungsten wire. An accelerating voltage (fixed amount of negative high voltage) is applied to the surrounding cathode cap. A small emission current is then applied to the filament to achieve the release of electrons. The point at which the gun achieves good thermal emission as well as an acceptable filament life is called the saturation point. The cathode cap ( also called Wehnelt cylinder) must be slightly more negative than the filament. A resister is located in the gun assembly and is controlled by a knob marked “bias”. It creates the difference in negative voltage between the filament and the cathode cap. This allows the electrons to collect inside the cap, forming an electron “cloud” . An Anode located below the gun assembly, is electrically at ground, creating a positive attraction for the negatively charged electrons, which overcome the negative repulsion of the cathode cap and accelerate through the small hole in the anode.

 

Glass lenses of course, would impede electrons, therefore electron microscope (EM) lenses are electromagnetic converging lenses. A tightly wound wrapping of copper wire makes up the magnetic field that is the essence of the lens. Surrounding these coils is a shroud made of a metal that will not hold a magnetic charge when the lens is shut off. The electron moves through the center hole in this solenoid. The electron path is further constricted by a brass lining inside this space known as the pole piece. The pole piece has a small gap within it at which point the beam is most influenced by the electromagnetic current. This is appropriately referred to as the pole piece gap.

Electron paths are usually represented by straight lines running through a convex lens. More accurately, however, the electron paths form a tight spiral as they are accelerated through the lenses. The path and trajectory taken by the electrons are influenced by the lens current as they pass though a small opening in the lens.

Cross over is the point at which the electrons converge. This defines the local length of the lens.

The condenser lenses in the TEM serve much the same function as that of the condenser in the light microscope. They gather the electrons of the first crossover image and focus them onto the specimen to illuminate only the area being examined. A condenser aperture is used to reduce spherical aberration. The Objective lens is used primarily to focus and initially magnify the image. The specimen stage is inserted into the objective lens for imaging purposes. A cold finger or anticontaminator also sits near the objective lens. It consists of a thin copper rod at liquid nitrogen temperatures, so that contaminants are attracted to it. The cold finger reservoir must be must be filled with liquid nitrogen before the microscope is used. Contaminants sometimes cause a phenomenon known as drift. Drift is the apparent "movement" of the specimen across the screen. It is caused by poor contact between the grid and the specimen holder causing a buildup of heat and static charges. An objective aperture is used to enhance specimen contrast. Intermediate lenses magnify the image coming from the objective lens. Finally, projector lenses further magnify the image coming from the intermediate lens and projects it on to the phosphorescent screen.