Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Heinrich Hertz (1857-1894) devised an oscillating electrical circuit ending in two metal spheres. The oscillations caused sparks across the air gap between the spheres, and Hertz was able to detect and map the electromagnetic fields generated, as predicted by Maxwell's equations. German and Czech stamps note the centenary of his research, showing the spark gap and the field lines.