As Ernst Ruska discovered, having an ingenious idea like the electron microscope can occur in the blink of an eye, but overcoming the finer details to create a successfully working instrument can take years.
Two incredible circles closed for Ernst Ruska in December 1986, at the age of 80. The first was that Ruska was finally receiving the Nobel Prize in Physics for his key role in developing the electron microscope several decades earlier. The second was that his dinner partner at the Nobel Banquet was Queen Silvia of Sweden, and they had more to talk about than just his scientific achievements – as a child Ruska used to play with the Queen’s father, Walther Sommerlath, in their hometown Heidelberg.
Sommerlath wasn’t the only playmate Ruska had in Heidelberg. Ruska also yearned to play with optical instruments, in particular the big Zeiss microscope that his father kept in his study. However, Ernst and his six siblings were strictly forbidden to touch the microscope, in case they damaged the lens or samples. Like all children told not to touch anything, this only made Ruska more eager to use the microscope, more so when his father, a passionate botanist and mineralogist, found the time to show his children magnified objects with the instrument. Ruska also had access to the telescopes in the observatory on Königstuhl, the hill overlooking Heidelberg. His uncle Max was in charge of the observatory and Ruska would visit him from time to time to see the telescopes in action.
Fascinated by hands-on-science rather than by theory, Ruska wanted to study engineering at a Technical High School, much to the dismay of his father, who regarded this as an inferior educational path for the son of a professor of the history of sciences. But Ruska stuck firmly to his course, went to Munich to specialize in electrical engineering, and then moved to Berlin.
In Berlin, Ruska worked on high-voltage techniques and electrical plants under the guidance of Adolf Matthias. At the end of the summer term in 1928, Matthias announced his plan to set up a small team to develop a high-performance cathode ray oscilloscope that could measure the voltages of very fast electrical processes in power stations and on open-air high-voltage transmission lines. Ruska jumped at the chance and became the youngest member of the team, headed by Max Knoll who would become an important and inspiring colleague.
In a cathode-ray oscilloscope, a heated metal plate, the cathode, shoots out a wide beam of electrons, and these are focused to form a dot, or writing spot, on a fluorescent screen. The more defined one could make the writing spot, by tweaking either the diameter or intensity of the dot, the better, so the team were looking to improve ways in which to sharpen the focus of the electron beams.
Ruska discovered that engineers had been using short coils carrying an electric current to concentrate electron beams for around two decades. But while he was scanning the scientific literature, he was amazed by a recently published paper in the journal Archives Elektrotechnik by a professor from Jena University called Hans Busch. Busch suggested that the magnetic field generated in the short coil of a cathode ray tube made charged electron particles behave in the same way that light does when it passes through a convex glass lens, such as the type of lens found in Ruska’s father’s microscope.
Light microscopes use two lenses to allow us to see what is normally invisible to the naked eye. As its name suggests the first lens, the objective, or object lens, receives the light rays from the illuminated object under scrutiny to produce a real image. This real image is magnified and viewed through the second lens, called the ocular. Together these two lenses can magnify objects to up to 1,000 times their size.
The idea that electrons could behave like light rays fascinated Ruska. Unfortunately, Busch’s theory was just that, a theory. Busch never tested his hypothesis, apparently abandoning it after comparing it with contradictory, older data from his lab. But Ruska thought that if a coil of the type that Busch described really did resemble a lens, then he should be able to obtain images from it – by using magnetic fields to converge electron rays and focus on a target.
In his graduate years, Ruska submitted a research thesis in May 1929, in which he calculated and tested the imaging properties of a magnetic coil. Using the coil, he gained the first recorded electron-optical images – of the anode aperture of the cathode-ray tube.
From here to the first electron microscope, however, was no straightforward task. After a failed approach to create a cheaper version of this experiment as part of his diploma thesis in 1930, Ruska returned to the original idea of using short coils as lenses. In an analogy to the light microscope Ruska built a tube with two short magnetic coils in a row. Although the apparatus could only magnify objects a modest 15 times, it was the first proof that magnified images could be obtained by electron beams and magnetic fields.
But there was a big drawback: the electrons emitted a huge amount of heat, enough to destroy the objects the apparatus was setting out to view. What use was creating images of such modest magnification, if even the hardiest metals were burnt to cinders, wondered Ruska.
Guided by Knoll, and supported by his fellow colleague Bodo von Borries, he set out to build a more efficient microscope. In the back of their minds they all knew that convincing people about the virtue of this new instrument would rely on meeting, and even surpassing the power and capabilities of the light microscope. The answer to this would involve a meeting of old and new theories about light.
The light microscope’s ability to be an extension of the human eye had stunned scientists since the end of the 16th century. However, there are limits to what can be seen, and these were finally explained and defined by the physicist Ernst Abbe in 1872. Abbe was hired by the company that had made Ruska’s father’s microscope, Carl Zeiss, to put the development of microscopes on a solid scientific ground. This was more difficult than he had anticipated, and, after some initial setbacks, he worked out a theory of how images are formed.
In short, Abbe found that the bigger the aperture, in other words, the angle through which the object lens of the microscope can accept light, and the lower the wavelength, the better the microscope can distinguish individual details within an object. But to reach what seems a simple conclusion required a deep understanding of the way light behaves in a microscope, and this could be described in terms of classical knowledge of the physics of light waves. The light waves that pass through the object under investigation in the microscope are diffracted and are therefore subject to interference on their way to the object lens. The greater the number of diffracted waves that reach the object lens, the better resolved the microscopic image is.
As creating an image depends on light being diffracted, Abbe could describe and define why light microscopes can’t see objects below a certain size. Diffraction occurs when light is allowed to pass through the spaces in an object, much like the way in which water waves pass through the spaces in between reeds. If a gap is smaller than the wavelength of light the waves cannot pass through, and therefore diffraction can’t occur. According to Abbe’s calculations on the wavelength of light, this meant the size limit in a light microscope would be about 0.5 micrometres, and so the magnification factor could not be much more than 1,000.
To see anything smaller would therefore require another form of energy that has a shorter wavelength. A solution would lie in a new, upcoming field in physics: quantum mechanics. In 1924, the French physicist Louis de Broglie devised a groundbreaking theory on materials that at first glance would appear to have little or no interest to engineers. de Broglie proposed the revolutionary thought that minute particles, such as electrons, could also behave like waves.
At first, de Broglie’s idea chilled Ruska to the bone. He feared that “even at the electron microscope the resolution should be limited again by a wavelength.” Imagine Ruska’s relief when he applied the equation de Broglie had devised to calculate wavelengths, and discovered that his electron waves would be at least an order of magnitude smaller than light waves. In theory, electrons should be able to see smaller objects than light.
Cap in Hand
In practice, though, constructing the first electron microscope with a larger magnification than a light microscope would require all of Ruska’s engineering skills. The trick was to get the design of the coil right. The shape of the coil needed to be able to compress the magnetic field to a tiny area running through the middle of the coil. This way the focal length could be kept as short as possible, which is a prerequisite for high magnification.
For this purpose, Ruska and von Borries constructed the so-called “pole shoe lens”, which is a coil encapsulated by iron with a narrow gap in the middle to help compress the magnetic field. This special lens was patented in 1932 and is still used in all magnetic electron microscopes today, and with it Ruska built an electron microscope in late 1933 that had a magnification factor of 12,000.
Despite this success, Ruska couldn’t convince industry to invest in the production of his electron microscope. The problem of overheating specimens had not yet been overcome. Ruska had discovered a partial solution to this in that very thin samples could be resolved by electrons through diffraction only, and not absorption, which means less heat is emitted.
However, this wasn’t enough to convince a sceptical industry to part with their money. “Who are the potential customers of such a device?” they would ask themselves. The obvious answer was biologists, but what did Ruska’s microscope do to biological specimens? The vacuum tube dehydrated them, the electrons damaged them, and the heat that was generated burnt them.
A familiar face, however, was convinced that biologists could study living cells under this microscope. Helmut Ruska, the inventor’s younger brother, who was now a medical doctor, was sure the microscope would work, but only if improvements were made. He managed to convince his former medical professor, Richard Siebeck, director of the medical clinic at the Charité Hospital in Berlin, that his brother’s invention was worth investing time and effort into.
In an advisory opinion to the industry, dated 2 October 2 1936, and three years after the completion of the prototype electron microscope, Siebeck described how Ruska’s electron microscope could be of immense value to him, as it could advance research into the causes of disease, particularly infectious agents that couldn’t be seen with a light microscope, such as those that cause smallpox, chickenpox, measles, mumps and influenza. “Success seems to me so close, that I am ready and willing to advise on medical research work and to collaborate by making available the resources of my institute,” wrote Siebeck.
This assessment was the carrot that helped convince industry to take on the economic and technical risks and invest in developing electron microscopes. In 1937, Siemens began developing Ruska’s microscope, and the first serially produced “Siemens Super Microscope” was delivered to the laboratories of I. G. Farben in Frankfurt-Höchst in late 1939.
The contributions of several researchers turned the initial concept of a microscope into a workable idea. But during the early days of his studies Ruska stood alone in his belief that an electron microscope could be developed. Not that the doubters hampered Ruska, in fact he would later view it as a benefit. “The doubt of the others has the advantage of leaving the field uncrowded,” he said in his Nobel Banquet speech. “Mostly, this is understood only much later, in the beginning one is very disappointed.”
Ernst Ruska website. http://ernst.ruska.de
Kruger, D. H., Schneck, P. & Gelderblom, H. R. Helmut Ruska and the visualisation of viruses. Lancet 355, 1713–1717 (2000).
Microscope. Encyclopaedia Britannica Online (2007). http://www.britannica.com/eb/article-9108723
Robinson, A. L. Electron microscope inventors share Nobel Physics Prize. Science 234, 821–822 (1986).
Ruska, Ernst: The development of the electron microscope and of electron microscopy. Nobel Lecture, December 8, 1986.
Urban, Knut (President of German Physical Society): Ansprache zur Einweihung des Ernst Ruska-Baus der Technischen Universität Berlin, November 24, 2005 http://www.dpg-physik.de