Cloud Chambers

For the emergence of cloud chambers as a method for quantifying ionization, we need to go back to 1898 when JJ. Thomson reported on his observations of ionization caused by X-rays in C. T. R. Wilson’s cloud chamber.  Thomson was trying to measure the electric charge of the electron as he reported in his paper “On the Charge of Electricity Carried by Ions Produced by Roentgen Rays” (pages 528 to 544 of The Philosophical Magazine, 1898).  This attempted measurement was the inspiration for Millikan’s oil drop experiments that by 1913 did get a measurement of the charge of the electron.  A lot of Thomson’s 1898 paper was devoted to describing how Wilson’s cloud chamber worked and how the condensation triggered by the ionization in the chamber could be used to measure the charge of the electron.

Of course in around 1900, no one was quite sure how X-rays and “ions” and electrons might be related (except via the aether, but that relation did not seem to convey much).  And so for over a decade, the cloud chamber was considered to be “detecting ionization” or “charge.”  But once C. T. R. Wilson started photographing tracks in the chamber from 1911 on, as Peter Galison says (p.114 Image and Logic ),  “All at once, Wilson’s chamber was all the rage.”

The first bit of rage concerned the “beta ray” ionization (electrons we would say) produced secondarily by gamma rays and x-rays.  Of course, gamma rays and x-rays themselves don’t ionize things directly and, it being 1912,  most researchers were unaware of Einstein’s 1905 discussion of light quanta and it was generally assumed that gamma rays and x-rays were either bound pairs of neutral particles or aetherial pulses.  Still, the tracks of beta rays demonstrably derived from gamma rays and x-rays seemed to suggest the presence or operation of something more particulate or corpuscular or more definite than aetherial wave pulsations, so at least we can now understand the rage part of the use of the cloud chamber – but, as Peter Galison brings out so marvelously in his book, things looked quite different to people at the time – which is, I think, the lesson this blogging business is teaching me over and over the hard way:  Things looked different to people at that time.

Or, to put it another way, up to 1911, the cloud chamber was not a particle detector, but, as I hinted above, a method for quantifying ionization.  Galison calls the application of the cloud chamber before 1911, “condensation physics” (p.136 Image and Logic) and notes that C. T. R. Wilson did not have to make detailed assumptions about the nature of the ionization involved.  But when Wilson began photographing the ionization (Galison says on page 139), “the sense and meaning of the chamber changed” as “the knotty clouds blended into the tracks of alpha particles and the ‘thread-like’ clouds became beta-particle trajectories.”  And the nature of the questions posed via the chamber changed to questions of gamma ray energies and particle scattering.  In Galison’s masterful treatment of the material culture of microphysics, the next chapter moves on to the use of photographic emulsions where particles are tracked directly in the transformations they themselves induce in the photographic material itself.  So on page 143, he quotes Wilson’s student, Cecil Powell, in 1959, saying that Wilson’s qualities “seem to gleam like the banners  of a vanished age.”  That age and its vanishing are the next topics in this blog for we have to look into the depths of the four decades between seeing beta-particle trajectories in cloud chambers and the systematic reading of photon energies in bubble chambers.  What would seem to be a logical progression of some sort turns out to be a twisted story indeed, or so it seems to me.  Fortunately, Galison himself offers some guidance in unraveling the story in How Experiments End, though it will take us some time to reach the beginning of even of that.