In his never-ending search on cyclotrons, their technology and history, Tim found this gem of a demonstration waiting to be re-discovered. Before the advent of numerical computers, accelerator builders pulled out their batteries, spools of wire, and ingenuity. Here is a paper by Lawrence Cranberg on Magnet Calibration by the Floating Wire Technique, a paper published by the US Atomic Energy Commission in 1951. The floating wire technique was used to visualize the paths (orbits) of charged particles in magnetic fields. A loop of wire, carrying a current will be drawn into a magnetic field (assuming the polarity is correct) and seek the center of the field, so as to include as many of the flux lines as possible.