The uses of the obsolete

Something that’s no longer used can still be illuminating.

Last week we noted that the reciprocating steam engine, although long superseded by the turbine form, still finds its place in textbooks because it is easier to picture and analyze.  Indeed, it remains useful for much of basic thermodynamics.  Pondering this, we came up with other examples along the same lines.

Our astronomer retains his undergraduate Electrical Engineering textbook, probably because he never felt he quite understood the subject, and thought it might be useful someday.  In that book is an extensive chapter on vacuum tubes, including details on how to design circuits that use them.  Now, even at the time tubes were obsolescent.  Although there were still devices around that used them, few if any new ones were being built, and the prospect of having to design a new tube circuit was remote.  But once one grasped the ideas of voltage and electron motion, tubes were fairly easy to understand.  Solid-state transistors are rather more difficult; indeed, one college Physics professor we know claims he still doesn’t understand them.  But they work kind of like tubes, so tubes provide a useful introduction.

One form of vacuum tube was the cathode-ray tube.  We haven’t seen one in action for decades, but once they were ubiquitous: just about every TV and computer in the 20th century used one.  They still exist in textbooks as a way to demonstrate electric fields and ion motion, a sort of counterpart to the theoretical piston of last week.

Much simpler is the magnetic compass.  We have two or three from our earlier days hiking in the wilderness.  There is a magnetized needle, free to swing on a pivot, so it aligns with the Earth’s magnetic field; centuries-old technology.  The magnetometer in your smartphone performs the same function in a much more complicated way, which we won’t try to explain to you.  (It is also, in principle, more accurate; we’re not sure how it works out in practice.)

But our photographer points out that old technology is not always simpler or easier to understand.  Film photography is based on a photochemical reaction: if light strikes a molecule of silver chloride or silver bromide, the silver is changed to solid silver (fine particles, which are black).  The nineteenth-century breakthrough was a way to have only some molecules change, where the light was most intense, and then to get rid of the rest of the silver halides. Except that it wasn’t that way for most of the analog era.  Instead, as with current film, light excites some molecules in a way that is only half-understood, so they are susceptible to being reduced to silver by another chemical called a developer.  Exactly how this happens, and how various chemicals in the developer work, is a matter of dispute among those still interested.  By comparison, the solid-state detectors at the heart of your digital camera are pretty straightforward.  (That is, if you ignore the software.  We think that the software in a current digital camera is probably too complicated for any one person to understand.)

Our navigator chimes in that maneuvering a sail-powered vessel is more complicated, and harder to understand, than simply setting a course and speed in a steam-powered ship (turbine or piston).  He’s done some of both, and holds in awe anyone who can manage a full square-rigged ship in a gale.

So obsolete technology can still be useful for teaching purposes.  But not always.