Ruska’s microscope

A former student recently alerted me on Twitter to a remarkable video. The grainy sequence shows a carbon nanotube inside of which suddenly appears a square array of dots that dances, buffeted by thermal motion, and grows steadily row by row to fill the tube. It is an extraordinary observation – the nucleation and growth of a crystal of sodium chloride imaged at atomic resolution, imaged using a transmission electron microscope, one of the truly transformative tools of the 20th century.

At the turn of the 19th century, scientists revealed the discrete, seemingly particulate nature of matter where everything was composed of discrete building blocks – nuclei, electrons, protons and neutrons – many of which could be deflected using electrical and magnetic fields. Among others, Johann Wilhelm Hittorf and Kristian Birkeland found that a circular coil could focus a beam of electrons down to a sharp point.

As cathode ray tubes began to be developed for display screen applications, work intensified. The physicist Hans Busch made calculations of the trajectories of electrons in a beam subject to such a cylindrical magnetic field. The result was a recipe for the construction of the electron equivalent of a convex lens. He demonstrated that varying the current changed the focal length of the lens. Although his paper marks the beginning of what today we call electron optics, the agreement of his calculations with his earlier experiments – more than a decade earlier – may have caused Busch not to follow the work up much further.