Faraday’s invention of the gas discharge tube had two major repercussions. One set of investigations, as we’ve seen, led to the development of spectroscopy. Another set led to the discovery of the electron.

In addition to the bright color of the gas in a discharge tube, Figure 37.4 shows a separate, constant glow around the negative electrode (i.e., the cathode) called the cathode glow. As vacuum technology improved, scientists made two discoveries:

1. At lower pressures, the cathode glow became more extended.

2. If the cathode glow extended to the wall of the glass tube, the glass itself emitted a greenish glow—fluorescence—at that point.

In fact, a solid object sealed inside a low-pressure tube casts a shadow on the glass wall, as shown in FIGURE 37.8. This suggests that the cathode emits rays of some form that travel in straight lines but are easily blocked. These rays, which are invisible but cause the glass to glow where they strike it, were quickly dubbed cathode rays. This name lives on today in the cathode-ray tube that forms the picture tube in older televisions and computer-display terminals. But naming the rays did nothing to explain them. What were they?

FIGURE 37.8 A solid object in the cathode glow casts a shadow.