Sunday, May 7, 2017
April 1, 1943: The Electron Microscope
Article on The Electron Microscope written by Opa in the Kansas State Engineer
The Electron Microscope
Magnification with Electrons
Modern Science, like modern armies, moves with blitzes, and any time of apparent dullness is bound to be crowned by some especially big and great invention or some revolutionizing theories. The electron microscope came to us as such a blitz, and its history is correspondingly brief.
Only twelve years ago, in 1931, C.J. Davisson and C.J. Calbick performed their experiments on the lens properties of apertures and developed the formula for the focal length of a circular and a split aperture. This gave birth to the predecessor of the electron microscope, the electrostatic electronic lens. Geometrical electron optics became a popular subject of research in the coming years, and two Germans, E. Bruecke and H. Johansson, have the credit of producing the first electron images of an oxide cathode with an aperture lens system utilizing 300-volt electron beams. After that, things started to move more rapidly. In the same year, M. Knoll and E. Ruska developed the first magnetic electron microscope and produced images of a cold cathode with short magnetic lenses, utilizing 60,000-volt electron beams. Since there was no literature on this field yet, E. Bruecke and O. Scherzer, in 1934, published the book which later became the classic of electron microscopy, “Geometrische Elektronenoptik.”
Skipping through the next years, we come to the research laboratories of R.C.A. and find a commercial electron microscope only 16 inches long and capable of magnifying 100,000 times.
The important feature of any kind of microscope is not, as is commonly believed, the magnification factor, but the size of the smallest detail visible. An analogy to this difference is given in the case of the photographic plate. If a picture is taken on a coarse-grain paper such that the details do not show up clearly enough, no amount of magnification will make the picture clearer. Additional magnification would only serve to make the grains stand out more. In microscopy, the wave length of the light beam – or in the case of the electron microscope, the electron beam – corresponds to the grain in photography. If a detail is smaller than one half of the wavelength applied to it, it is not perceivable, no matter how strong a magnification is used.
This fact sets a theoretical limit to the results obtained with a light microscope. Since the wavelength of light is about 1/50,000 of an inch, the smallest detail resolvable by a light microscope is approximately 1/100,000 of an inch, linear. The present light microscopes, therefore, can theoretically be improved to give an amplification such that the particles of 1/100,000 inch length can be made visible, but no amount of magnification will make a particle visible which is smaller.
Since the electrons may be thought of as a wave motion of comparatively short wave length, electron beams can be used in a microscope just as well as light beams. The wavelength of the electrons depends on the potential with which they are accelerated, as was shown by de Broglie about 20 years ago. If the electrons have been accelerated through a potential difference of 50,000 volts, their wavelength is reduced to only a five-billionth of an inch. Therefore, details can theoretically be made visible which have a linear extension of only two and a half billionths of an inch. It might be figured, thus, that an ideal electron microscope is 25,000 times better than an ideal light microscope; in other words, that the magnification of the electron microscope to the light microscope corresponds to the magnification of an ordinary laboratory-size light microscope to the naked eye.
Naturally, there exists neither an ideal light microscope nor an ideal electron microscope. At the present time, the light microscope comes much closer to its ideal condition than does the electron microscope; the latter one, however, is still in the infant stage and has far to go.
In a light microscope, the light beams are focused by glass lenses. No glass nor any other matter will focus or refract electron beams; the only effective system of control for electrons is an electric field. Refer to the figure. A divergent electron, originating at P. enters the magnetic field of the solenoid S, and is thereby brought to a focus at P. Thus, a solenoid or some other source of an electromagnetic field serves as a convergent magnetic lens. By the aid of these magnetic coils, a system of magnetic fields can be set up which corresponds to the system of glass lenses in the light microscope.
Since electron beams, as such, possess a wave length outside the range of the human eye, the projected picture is not visible immediately. To make it visible, the electrons are either made to fall on a photographic plate, or, if immediate vision is desired, on a fluorescent screen.
The proper preparation and proper arrangement of the object, which is a critical problem in the case of the ordinary light microscope, becomes even more critical in the electron microscope. The object is usually supported on a very thin celluloid film, about one two- millionths of an inch in thickness. The specimen is prepared as a suspension in water. A droplet of the suspension is placed on the celluloid film, and, after the water has been permitted to dry, the specimen rests on the celluloid film as an extremely thin layer. The maximum thickness which permits electrons to pass through the specimen, depends on the substance of the specimen and on the voltage applied to accelerate the electron beam. A voltage of 200,000 volts makes about all substances, which are ordinarily used, transparent. It is obvious that a higher voltage reveals more details, until a point is reached where the picture becomes faint, or, to use a photographic expression, overexposed. Increase in voltage reveals more details until a voltage of about 100,000 volts is reached; the picture at 200,000 volts is no longer usable for the transparency has become too great.
An electron microscope of the type described above can be used whenever it is not necessary to observe surfaces of opaque specimens. Electrons reflected on the surface of these specimens cannot be utilized, because the velocity of reflected electrons is too inhomogeneous to lead to satisfactory pictures.
This problem waited for its solution until the development of the “scanning electron microscope” had been successful. This microscope differs greatly from both the light microscope and the standard electron microscope. The picture of the object is now formed instantaneously, but, similar to television, the intensity of a single minute picture is recorded at one time. The final picture is built up of a great number of such elements of different intensity. Usually, the scanning electron microscope uses electrostatic lenses instead of electromagnetic ones.
The electrons are guided through a series of these electrostatic lenses and are focused on the object, where they form a spot less than a millionth of an inch in diameter. This concentrated electron beam causes a secondary emission of electrons, and emission which is proportional to the relative brightness of the part of the object hit. The returning secondary electrons pass through the last electrostatic lens and fall on a fluorescent screen. The electron beam will yet be too weak to be seen on the screen with the naked eye, so the light emission from the screen is used to control the output current of an electron multiplier. This output current can be amplified to any desired value and is then used to control the intensity of the halftone lines of the final image.
The question of the kind of lenses to be used in the commercialized electron microscope gave rise to much controversy within the manufacturing companies. In the late 1930’s, most builders had used electromagnetic lenses. A.E.G. in Germany, however, had done research on both methods and concluded that the electrostatic method gave more satisfactory results. The General Electric Company, which built the recent “portable” electron microscope, uses electrostatic lenses in it, while most R.C.A. models employ the electromagnetic type.
The electromagnetic lens consists of an iron-core coil wound around the axis of the electron microscope. Its poles have to be shaped so as to give the desired field distribution to concentrated flux gaps. The coil does not have to be in the vacuum to which the electron beams are restricted, thus the physical manipulation and adjustments of the coil are made very convenient. The voltage applied to the coil is comparatively low, such that the insulation problem is of no importance. The focal qualities of the lens can easily be varied by varying the current through the coil.
The main disadvantage of the electromagnetic lens system lies in the difficulty of alignment. The lens has to be realigned for every change in lens current, beam characteristic, or the electron accelerating potential. In order to keep the image from blurring, voltage and current regulations of one part in 50,000 are usually necessary.
The electrostatic lens consists of three apertured discs, the center one of which is insulated from the outer two and held at a different potential. The focal characteristics of such a lens is varied by adjusting the potential on the center disk. This system has a definite advantage over the electromagnetic system, because a change in the accelerating potential will cause a corresponding and proportional change in the lens potential. Thereby, the focal properties of the lens system remain unaffected and realignment is not necessary. Current and voltage regulations are no longer of great importance, and an ordinary half-wave power supply is sufficient.
Because of these advantages, the General Electric Company chose the electrostatic system for its commercial electron microscope. This system permits a simpler power supply, easier and more permanent adjustment of the electron beam and an all-around more convenient manipulation.
The applications of the electron microscope are as varies as can be expected. Medecine, chemistry, biology, botany, physics, are just a few of the fields of its widest applications. At the present time, the medical field has taken advantage of the electron microscope more than any other scientific field. Many bacilli and viri, hidden from the human eye in the past, had to surrender its disguise in front of the strong penetrating power of the electron microscope.
Very extensive research has been done with the electron microscope in the field of chemistry, a notable example being with chemistry of smokes and dusts. The individual particles of these substances are so small that they cannot be observed with the light microscope. The electron microscope revealed that most smokes, especially metal smokes, have particles of a typical, characteristic shape. The effect of finely divided carbon, so-called carbon black, as a reinforcing agent for rubber had been known before, but it took the electron microscope to find out the cause for this behavior.
The electron microscope is still very young, but at this time it has rendered enough valuable service to the field of science and the field of human welfare, that its development and its future are well worth watching and promoting.
Wow. If you read that entire article, I'm clapping for you. This is where I discover that not everything Opa wrote is interesting to me. This was VERY boring to me. However- I put it here because not everyone is bored by electron microscopes in the 1940s. If you enjoyed this, (and I know one person who will) then I'm so glad.
I will say, it reminds me of how important it is for people to do what they are good at. I have zero interest in the development of electron microscopes, but I am invested in the technological advances that have been made as a result, particularly medical ones! So all you nerdy engineers out there- thank you. Thank you for doing your thing and making the world a better place, even if a lot of us are bored out of our minds about the process. Don't worry about that- you just keep going. I'll write you a story, you build me a microscope.