Life and Work of Rolf Wideröe by © Pedro Waloschek,     => Contents

2  Karlsruhe - the Ray-Transformer

Karlsruhe's Polytechnic, known as the `Fridriciana', is probably the oldest in Germany and has a very good reputation. Heinrich Hertz was one of many who had worked and taught there. I estimate that, in my time, there were about three to four thousand students in Karlsruhe. We cannot therefore regard the German Universities of that time as the student factories we know today, where student numbers of 20 to 30 thousand or more are the norm.

The relations between students and tutors were excellent and of a very cooperative nature during my time in Karlsruhe. I especially remember Professor Schleiermacher who taught us theoretical electrical engineering. He was a friendly old man. We also had a very fine mathematics professor called Böhm.

Professor Wolfgang Gaede taught us physics; he was one of the high gods and a little more distanced from us students. However, as mentioned previously, it was all very harmonious and we had no problems.

I found the teaching first-rate and well balanced. Professor Richter's lectures on the theory of electric machines were much influenced by the practical facts of engineering. We learnt a great deal about direct current machines, commutators and similar things which have now almost completely disappeared. We also had exemplary teaching in mathematics, chemistry and physics. Overall, it was pretty well balanced and had an academic flavour. It contained much more than just the purely practical aspects of engineering.

Spannhake, a teacher of worth, taught us about hydroelectric power machines. He was of a more practical bent. Professor Tolle taught us technical mechanics and he was very good, and Professor Nusselt was our thermodynamics lecturer.

The most important part were the lectures. We didn't have special seminars for our free subject, instead we would have a lecture on, for instance, Einstein's theory of relativity. The laboratories too were excellent. For our laboratory work we would be divided up into groups and given practical problems which we had to solve under the supervision of assistants. We worked quite independently. Later on we also had to design and build electrical machines. Our education was versatile and of good quality.

However, it was a shame that I no longer had the opportunity to study more physics. During my time in Karlsruhe, collaboration and communication with the physicists was not as good as it is today. There were few conferences, symposiums or meetings, and I also had very little personal contact with the physicists. Lectures on physics (Gaede) were of course included in our course, but we did no practical work.

It was also in Karlsruhe that I wrote my first publication - on a subject which has nothing to do with engineering. Inflation was rampant when I went to Germany in 1920; the value of the German Mark was constantly dropping. Price increases caused everyone to be interested in economics, and I would therefore make a daily plot of the US-dollar rate. This was for purely practical reasons. My father had initially bought me German Marks and now I wanted to know the best time to change money again.

This resulted in a dollar curve which, drawn on logarithmic graph paper, reached from the floor to the ceiling of my room. At first the dollar equivalent rose at a more or less linear rate, although naturally with major fluctuations, but by the end, in 1923, the exchange rate increased in such an alarming way that I had to use double-log graph paper. While one US-dollar had been the equivalent of 192 Marks in January 1922, by the end of 1923 it was about 4,200,000,000,000 Marks! This curve prompted me to write an essay for the Norwegian State Economics Magazine which was published in 1924 [Wi24]. I didn't take much notice of such things later on, but it was my very first publication.

Karlsruhe had a Nordic Club. Quite a few Norwegians and Swedes as well as a few Finnish students (Swedish and Finnish Fins), frequented this Club. There was also someone from Iceland and a Dane, Mr. Hansen. We often held parties as there were many National holidays to be celebrated and there was much Cognac and Swedish punch to be had.

Some of the names have stayed in my memory; a Norwegian called Rotheim. He was the inventor of the spray-box, but his sole reason for inventing it had been to spray wax on skis. When he returned to Norway some time later, he had a batch of these spray-boxes manufactured. He had them patented as well, but it was not an economic success. He died quite young.

I also remember Jack Nilsen, a Norwegian tennis champion. He later became head brewer at Ringness. I bought his bicycle when he went back. Grude von Stavanger was a great baritone. There was a student of architecture called Björnson-Langen. His mother, the daughter of the Norwegian poet Björnstjern Björnson, had once been married to the publisher Langen (Simplicissimus) in Munich. He was great fun. And there was also my good friend Kaare Backer, he became a construction engineer, is still alive and over 92 years old. I went to visit him in February 1991 on the occasion of his diamond wedding anniversary.

I did a month's work experience in Strasbourg, in an electric motor factory. I had to wind the coils of a motor, a difficult task, and then I had to go to work outside, to connect various electrical cables onto a mast.

My diploma-dissertation, completed in 1924, was concerned with `Potential Distributions in Chain Isolators' for high tension lines. This involved various problems. We had a tutor in high voltage technology, Professor Bonte, who had written a book which included several of his calculations for electric potentials. I had discovered that one of the calculations was wrong. This was the starting point for my dissertation, and I corrected his mistakes. I remember that I used differential calculus, but I also wanted to investigate the matter experimentally. I built a model of an overhead pylon at scale 1:100 with some suspended isolators and put it in a bath tub which I used as an electrolytic tray. As far as I can remember, this method was already known at that time, and this is how I was able to measure the voltage distribution in water. After I had solved a few problems of surface resistance (silver electrodes), the thing worked quite well. This work was awarded with a 5.9 (6 was the top mark).

I had a lot of help when I was working on my dissertation in Karlsruhe. Existing equipment was made available to me, and I was allowed to use the workshop. Whenever it appeared necessary, my deadlines were extended.

In the autumn of 1922, while in Karlsruhe, I had already developed the basic ideas for a `ray-transformer'. This machine would accelerate particles as if very high electrical voltages were available, but without the need for such dangerously high voltages, which could not be achieved in practice anyway.

The question I asked myself at that time was whether electrons in a ring shaped vacuum chamber would behave in the same way as if they were in a copper wire of an ordinary transformer's secondary coil. When the electric current in the primary coil changes, they should really be accelerated in the same way as the electrons in the transformer's secondary coil.

For example, if the alternating current in a transformer's primary coil changes direction 50 (or 60) times a second, this produces a force on the electrons in the secondary coil which `accelerates' them in either direction. A single acceleration in one direction therefore happens within a fraction of a second and this was exactly the effect I wanted to exploit.

As the electrons were no longer confined within a copper wire, I had to switch on an appropriate magnetic field to keep them on a circular orbit. This magnetic field would, nevertheless, have to adapt itself to the increasing velocity of the circulating particles.

If there is a sufficiently high vacuum in the tube (imagined as the transformer's secondary coil), there should be hardly any electrical resistance and the electrons would achieve an extremely high speed within a very short time. This would correspond to the acceleration produced by a very high voltage. It was not so easy however to calculate the speed reached by these electrons. I was soon convinced that the electrons would not take long to come close to the speed of light and that the formulas of classical mechanics would therefore no longer apply.

In those days, people were still not quite sure whether the formulas contained in Abraham's absolute-theory were correct or those of Einstein's theory of relativity. Because of this, I initially calculated the movement of the electrons in the ray-transformer on the basis of both theories. Later on I used only Einstein's formulas, as, in the end, these did appear to me to be better.

I came to the conclusion that acceleration within one rise of the current, that is, within less than a hundredth of a second, would be equivalent to a `potential kick' of several million volts. The relatively small kicks at each revolution just kept adding up, and eventually resulted in this high number. It really was an amazing result, as this meant that the size of a machine which could reasonably be built would be quite modest; the electron orbits would be approximately 10 to 20 cm in diameter, if one were to use the technology for building transformer magnets which was available at the time.

In my first sketch (Fig. 2.1) I simply placed a flat (evacuated) accelerator vessel between the poles of a magnet [Wi23]. For this device I calculated the attainable energy. In a slightly later drawing (Fig.2.2) I took into consideration that a second, independent, magnetic field is required to guide the electrons on reasonably steady orbits. This second magnetic field is induced by a second coil which can clearly be seen on the drawing.

After thinking it over for some time I arrived at the conclusion that there is an important relation between the accelerating field (of the transformer) and the deflecting or steering field (for the circular orbits), which must be maintained over the entire accelerating process - if one wishes to sustain the same size of the orbit during the whole acceleration process: The mean field within the circular orbit (that is, the `accelerating' field) should always stand in a very particular ratio (precisely 2:1) to the deflecting field. This relationship, which later came to be known as the `Wideröe relation', even permits both fields to be produced by the same primary coil, which again simplifies the whole machine. The magnet's yoke would be similar to that of an ordinary, largish transformer-yoke and could therefore, if the pole pieces had the right shape, provide both the accelerating and the steering field simultaneously. However, I had not quite got this far with my first ideas in Karlsruhe.

In the end, I had spent so much time thinking about the principle that I was convinced that it had to be correct _ and this is really the crucial point: It is possible to accelerate particles with changing electromagnetic fields without using any static high voltages.

Until then the energy of charged particles had always been `accumulated' by means of (static) electrical fields. Therefore, more and more `volts' were required to achieve greater energy. What happens in a ray-transformer is, however, quite different and was quite new. Here the energy is accumulated in the form of kinetic energy, it can be increased without requiring high voltage. And this, in my view, was the important and basic idea for all further developments in this field and also for the entire particle accelerator technology which came later.

I didn't speak with anyone about my ideas and calculations in those days, because I realised that the 5th semester was too early to continue any work on this subject. I made a few notes on this in March 1923, which are still conserved in my copy-books [Wi23]. About half of my texts are in Norwegian, the rest are in German. However, after writing down these notes I put them on ice and continued my studies. I intended to proceed with this matter only at a later date.

At that time I knew nothing of what was going on in other laboratories, such as in England or Germany, where research on nuclear physics was being done, but I must have continued to ponder Rutherford's nuclear reactions and the possibility of creating better experimental conditions for them. In any case I wrote in one of my notebooks that one "would require at least 10 million volts and considerably more" to smash heavier atomic nuclei. Rutherford's alpha particles attained a maximum of about 10 MeV. Furthermore, one would have to be able to shoot, under controlled conditions, a far greater number of particles onto the atomic nuclei to be smashed.

In my opinion, building a device similar to a ray-transformer was the only way to accelerate particles to much higher energies or, using the language of that time, to achieve the appropriate `high tensions or potentials'. The energy of the particles was already then referred to as the `potential' which would be required to accelerate them to that extent, independently of the actual method used to accelerate them. This is exactly where the energy units which are still used today, the electronvolt (eV), the kiloelectronvolt (keV) and the respectively higher ones (MeV, GeV and TeV) stem from.

Then I took a description of the ray-transformer to a patent office in Karlsruhe and requested that they apply for a patent based on my notes. However, I heard no more from them and when the work for the ray-transformer in Aachen started to go wrong I wrote the whole thing off. Many years later, it must have been in 1943, during the War, my travels took me back to Karlsruhe and when I searched for the patent office I discovered that the entire neighbourhood in which it had been located no longer existed.

In 1924, after finishing my dissertation and examinations in Karlsruhe, I returned to Norway where I first completed my practical work, which consisted of six months working in the locomotive workshop of the Norwegian State Railways. I also did my National Service in 1925 during which I commanded six men and a farmer with a horse and cart for 72 days! It was a wonderful summer. I came back to Karlsruhe during the autumn of 1925. First of all, I compiled all my ideas and calculations on the ray-transformer and took them to Professor Schleiermacher who, as already mentioned, taught theoretical electrical engineering. He was very nice to me and carefully read my manuscript. Then he said to me, "Your entire thesis is right here". He had given the whole thing a very positive verdict.

Then I went to see Professor Gaede who was responsible for physics, and showed my manuscript to him as well. When I returned to him a few days later I was rudely awakened. He believed that my proposed apparatus would never work and told me that I should forget all about it. Even with the best vacuum achievable at that time (I guess it was about 10^-6 millibar), so many gas-molecules would be left over that the electrons on their long journey (covering several million kilometres in the small chamber!) would be absorbed far too quickly - far quicker even than it was possible to accelerate them. Of course, this was very sad for me and I was extremely disappointed.

However, I knew where I could find out more about the problem of electron absorption in gases. Professor Phillip Lenard, who had already been awarded the Nobel Prize in 1905, was working in Heidelberg at the time. His investigations were described in a book entitled `Quantitatives über Kathodenstrahlen aller Geschwindigkeiten' [Le18] which I found in the library. Lenard had measured the scattering and absorption of electrons of several energies (from ten to one million electronvolts) in layers of matter, especially in air. I drew the results of his measurements on logarithmic graph paper and found a beautiful curve for the absorption as a function of the electron energy.

Accordingly, Gaede's assumptions were wrong. The losses due to absorption quickly decrease at higher electron energies (somewhere above 400 electronvolts) and after that they hardly matter any more. Yet this does result in a lower limit for the beginning of the acceleration in the ring; that is, a certain minimum of energy is required to inject the particles.

I did not, however, go back to Gaede. I had come to the conclusion that my original idea of writing a thesis in Karlsruhe was no longer feasible. My aim had been to build a ray-transformer, or at least an accelerating tube. Gaede would not have permitted me to do this. After thinking about it for a little longer, it also seemed to me that the technology available in Karlsruhe was not sufficient for my plans.

I used to like reading `Archiv für Elektrotechnik'. In this magazine Professor W. Rogowski and Dr. Flegler had published papers describing their research work, for which they used very fast cathode ray oscilloscopes they had developed in Aachen. In their laboratory high frequency and high vacuum technologies were nurtured and therefore it was the right place for me. I wrote a letter to Professor Rogowski and asked whether I could work with him in Aachen and I received a warm reply. He wrote saying that he would be going to Switzerland for a holiday on such and such a date, and that he would be passing through Karlsruhe on the way back, "Join me on the train. We can travel together to Mannheim and you can explain it all".

I followed his instructions and we travelled to Mannheim together. The journey took about one hour. I don't believe that he understood much of my explanations, but I mentioned several times that I wanted to build a `transformer' for six million volts, and that must have hooked him. He was ambitious and always wanted to be just that little bit in advance of the competition. Thus he said, "This sounds very good, come to Aachen and we'll sort it out".

So I moved to Aachen. On the eve of my departure we had a tremendous party. It ended with us hanging all the chairs on the wall. In the middle of the night, or rather in the morning, I rode off on the train. My landlady was appalled when she saw the state of my room, but my friends ironed it all out again.

I was well received in Aachen. I registered with the Polytechnic, was able to sit in on a few lectures and worked in Rogowski's laboratory.