No, this is not about Einstein’s achievements as a moonlighting scientific paradigm shifter, while working as a patent examiner in his day job.

Einstein is famous for the theories of special and general relativity, and for the correct explanation of the photoelectric effect that has been rewarded with the Nobel prize. It is not so common knowledge that he contributed to the theory of Brownian motion, and found a new way of deducing Max Planck’s famous formula for the intensity of blackbody radiation – a prerequisite for an important invention of the 20th century: the laser.

I had graduated in physics, but I was ignorant about Einstein being an avid inventor himself until I read this biography by Jürgen Neffe and Shelley Frisch.

Einstein had spent part of his youth living near or literally in the electrical engineering company operated by his father and his uncle. As a child Albert Einstein became familiar with resistors, magnets, capacitors, light bulbs, generators, and engines.

Later Einstein worked on improvements of a gyrocompass as a technical expert in a patent dispute,  and together with his student Leo Szilard he filed 17 patents in German and some international patents on a refrigerator based on gas absorption. Besides, Einstein invented an automatic camera, and he even tried to optimize air plane wings and torpedos (the latter with not too much success, according to Neffe).

The refrigerator patent has been recovered at the end of the 20st century and prototypes have been built recently.  An excellent account of the history of the invention and its inner workings can be found on  Cocktail Party Physics, and the device is analyzed in all thermodynamic details here.

So Einstein’s early exposure to engineering gadgets might have triggered a live-long interest in “building real stuff”. Nevertheless, one might also trace the inventor’s spirit in his theoretical works: Remember the thought experiments used to explain relativity – a world comprising trains, mirrors, clocks, space ships, and falling elevators. A deterministic thinker’s paradise?

Could we dare to speculate that his life-long qualms with the strange nature of the quantum world is due to his engineering mindset (despite the fact that Einstein laid part of the foundations of quantum mechanics)?

The authors of the biography quoted above assume that Einstein’s informal training in engineering prepared him well for his future job as a patent examiner. He was able to nail down the issues with machine defying the laws of physics . If this is true, the process of issuance of patents has changed considerably. I have learned that patents are not granted primarily for the fact that devices do indeed work but for novelty; so it is OK to invent an alleged perpetuum mobile as long as its design is novel. But I digress.

I am finally happy to add Einstein to the list of physicists who contributed to fundamentals of physics and who worked hands-on. I remember Enrico Fermi being often called the last physicist who was both a theorist and an experimental physicist.  And there is of course Richard Feynman – remember the O-ring demo.

Back to the famous fridge:

It always takes me some time to wrap my head around cooling machines that do not use mechanical compressors. Fortunately, I found this annotated drawing of the Einstein-Szilard machine (Wikimedia, public domain):

… which is explained in detail here. [Edit 2017: Linked to archive.org as original server / site does not work anymore.]

How does it work?

Generally, any fridge or heat pump (regardless of process details) is based on closed cycle process including

• condensation of a refrigerant at high temperature and high pressure – at the rear side of the fridge or when a heat pump transfers energy to the water used for heating
• evaporation at low temperatures and low pressures – inside the fridge or when a heat draws energy from the heat source.

The pressure difference can be due to a mechanical compressor driven by electrical power. But instead of utilizing the total pressure of a single refrigerant, the partial pressure of a gas in a mixture of gases will do. Ammonia can be use used as a refrigerant, and its partial pressure is increased by evaporating ammonia, heating a mixture of ammonia and water. So the energy needed to drive such an absorption process is heat, not electrical energy.

So why is the Einstein-Szilard process even more complicated and based on three substances – ammonia, butane, and water?

Butane is the actual refrigerant, thus cooling is done when the partial pressure of butane is reduced, and butane evaporates at this reduced vapor pressure (in the Evaporator). The total pressure is nearly constant in this process: The partial pressure of butane is reduced by adding ammonia (from the Generator).

In the Condenser the butane partial pressure is increased by removing ammonia from the vapor. This is where water is needed: Water is sprayed into the ammonia-butane vapor, and ammonia is absorbed by the water droplets (due to its large affinity to water). There are two mixtures of liquids in the condenser: butane-ammonia and ammonia-water, and they are separated due to differences in density.

Finally: where is the external energy (heat) fed into the process? Heat is required to expel ammonia from the ammonia-water mixture (in the Generator) that is collected at the bottom of the Condenser.