Maltese Cross CRT

cathode rays - electrons - magnetic fields

What it shows:
Cathode rays are seen to travel in straight lines by casting the shadow of a Maltese cross on fluorescing glass. They can also be deflected by magnetic fields, as seen by the distortion of the shadow.

How it works:
Electrons accelerated down the tube are carried by their momentum to the front wall, where they strike the glass causing it to fluoresce. However, those electrons on a collision course with the cross never reach the glass, leaving a dark shadow in the green fluorescence Showing the effect of a magnetic field on the cathode rays, a magnet brought close to the tube deflects the electrons, distorting the image of the cross.

figure 1. Schematic layout of Maltese Cross CRT

Setting it up:
The Maltese Cross tube uses a 6V DC supply and induction coil. 1   A small Alnico bar magnet is sufficient to deflect the beam.

Comments:
On some Maltese Cross CRTs, the metal cross itself acts as the anode. The effect of the electrons being decelerated by the glass are shown in Bremsstrahlung. Rating ***


1 Ours are CENCO although are no longer available. Sargent-Welch model 2392A are similar