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Einstein once got involved in true experimental physics. That is, he designed his little machine (Maschinchen, in German, a word that makes me laugh; Einstein's Maschinchen...) to detect small potentials. Voltages, which are so easy to detect nowadays.

For as long as the name Einstein is remembered, it will surely be as a conjurer of great ideas, relativity being foremost among them. But the most celebrated theoretical physicist of all time also tried his hand once at designing an experimental instrument. It didn't amount to much, but that doesn't make it any less fascinating to physicists, who have now reconstructed the device that Einstein called his "little machine." In his biography of Einstein, the late physicist Abraham Pais asserted that the old man's biggest gift lay in an unprecedented understanding of statistical phenomena, which, for one thing, allowed Einstein to prove the existence of atoms by analyzing the zigzag Brownian motion of particles in water. Two years after that breakthrough, in 1907, Einstein applied a similar statistical perspective to electrons in a capacitor, a simple device for storing electrical charge, says physicist Danny Segers, director of the Museum for the History of Sciences in Ghent, Belgium. Einstein reasoned that when a capacitor is allowed to discharge, it should still contain a small voltage in the vicinity of one half millivolt. Today a voltmeter from the hardware store will tick off fractions of millivolts, but a century ago it was impossible to measure voltages that small, Segers explains.

So from 1907 to 1910 Einstein worked on a design he devised for amplifying a voltage--his Maschinchen, or little machine, as he would later refer to it. His friends Paul Habicht, an engineer and instrument maker, and Paul's brother Conrad helped him perfect the device, three copies of which survive in museums.

"When you look through the literature you don't find any measurements done on it, so we had doubts whether the machine ever worked," Segers says. Wanting to test one directly, Segers and his colleague Jos Uyttenhove built their own Maschinchen, based on a schematic published by the Habichts and an interior view of one of the museum pieces. They assembled a stack of six stages, or groups of metal plates, housed in a cylinder. In the first stage a plate slides past the discharged test capacitor, picking up a charge induced by any residual voltage left in the capacitor, and deposits this amplified charge in the next stage, where the process repeats to create a measurable signal.

The workings of his Maschinchen remind me of the workings of a photon multiplicator tube (in 1930 invented by the Russian physycist Kubetski). Are these related? Did Kubetski or people around him know about the little machine (there was no internet yet)? Or is the construction for amplifying small amounts bound by certain rules that lead to the desired outcome? Then again, I can't imagine that Einstein knew of such rules. How did he arrive at his idea?

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  • $\begingroup$ did you mean photomultiplier tube ? $\endgroup$ Jul 29 at 12:59
  • $\begingroup$ @J.W.Tanner Yes. Its structure is similar to that of the little machine. $\endgroup$ Jul 29 at 13:17
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No, Einstein's maschinchen is not much like a photomultiplier tube. It's much more like a Van de Graaff generator.

A photomultiplier is an example of an exponential cascade amplifier. A photon hitting the first surface will (typically with a probability of about 10 to 50%) knock loose an electron. The electron is accelerated toward the next stage, where it knocks loose more than one electron (on average). The secondary emission ratio, as this is known, can be in the range of 10 to 20. The process continues, with each stage increasing the number of electrons knocked free, and hence the current. At the end of the cascade, a relatively large number of electrons is released, which are enough for ready measurement.

The maschinchen seems to more like a Van de Graaff generator, where a moving element picks up electrons and transfers them to a different location. Multiple elements produce a higher voltage, but there is no change in the number of electrons. In a Van de Graaff generator, this is done in a single pass, using a flexible belt to transfer electrons. The accumulation of electrons produces a high voltage. The Van de Graaff was used to provide high voltage for early particle acccelerators (called "atom smashers" in the popular press).

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