In 1879, Crookes had reached the conclusion that cathode rays must be particles of some sort, observing that they traveled in straight lines and were stopped by metallic objects in their path. As a demonstration, Crookes inserted an electrode shaped like a Maltese cross in the tube, and it cast a sharp shadow on the fluorescence at the end of the tube (Figure 43a). Maltese cross tubes are still commonly demonstrated in university physics classes. As shown in Figure 43b, we built our own version using a more jovial target.
Figure 43 In 1879, Sir William Crookes inserted a Maltese cross–shaped anode into a CRT (a). The shadow on the fluorescent screen demonstrated that cathode rays travel in straight lines. Crookes also saw this as evidence that cathode rays must be some sort of particle that does not penetrate metal. (b) Our version of the Maltese cross tube uses a more jovial subject to cast a shadow.
We soldered a thin piece of brass to a 3/4-in. copper washer. We cut out the happy face on the brass sheet, and soldered a brass screw to a tab we left on the brass sheet. We painted the happy face with some leftover phosphor so we can see when the cathode-ray beam hits it. We then placed the happy face inside the Ace 1,000 mL Gledhill flask (Ace catalog number 14205–09) on which we had deposited a fluorescent screen (Figure 40). We carefully inserted the brass screw into a threaded 1/4-in.-OD × 4-in.-long spacer (made by joining two 1/4-in.-OD × 4-in.-long spacer) that we introduced through the Ace-Thred #7 port on the Gledhill flask.
We coupled the flask to the electron gun of Figure 41 with an Ace-Thred #25 coupling (Ace catalog number 5841–16). We connected the negative high voltage of our power supply (Figure 39) to the gun’s cathode (the 3/8-in. aluminum rod), and the ground terminal to the vacuum manifold and the happy face support.
As you can see in Figure 43b, the shadow produced on the fluorescent screen is very sharp, indicating that the cathode rays travel in a straight line from their source (the small hole in the anode cap) to the screen. They are intercepted very cleanly by the happy face mask. Moving a magnet about the CRT deflects the cathode rays. Depending where the magnet is placed, it may shift the illumination of the mask, or distort the shadow on the screen.
Crooke’s argument did not convince everyone. Most notably, it was rejected based on the results of an experiment conducted by Hertz in 1891. Hertz reasoned that the cathode rays should be deflected by an electric field if they were beams of negatively charged particles. Hertz placed two electrode plates within a CRT, and expected the beam to deviate when he applied a high voltage between the plates. The beam didn’t move. The cathode rays were not deflected in the way that would be expected of electrically charged particles. We know today that cathode rays are deflected by an electric field, and that the reason Hertz didn’t detect the deflection was the vacuum in his tubes was not sufficiently low to prevent interactions with the gas. However, Hertz didn’t know this, and accepted his experimental results.
In a different experiment, Hertz placed a thin metal foil in the path of the rays, yet still saw some glow in the fluorescent screen behind it. Unknowingly, Hertz was producing X-rays that excited the screen’s phosphor. The incorrect interpretation of these experiments convinced Hertz that cathode rays were not streams of particles, but rather some type of wave.
Leave a Reply