The most systematic studies on the new cathode rays were carried out during the 1870s by the English scientist Sir William Crookes. Crookes devised a set of glass tubes, such as the one shown in FIGURE 37.9, that could be used to make careful studies of cathode rays. These tubes, today called Crookes tubes, generate a small glowing spot where the cathode rays strike the face of the tube.

FIGURE 37.9 A Crookes tube.

The work of Crookes and others demonstrated that

1. There is an electric current in a tube in which cathode rays are emitted.

2. The rays are deflected by a magnetic field as if they are negative charges.

3. Cathodes made of any metal produce cathode rays. Furthermore, the ray properties are independent of the cathode material.

Crookes’s experiments led to more questions than they answered. Were the cathode rays some sort of particles? Or a wave? Were the rays themselves the carriers of the electric current, or were they something else that happened to be emitted whenever there was a current? Item 3 is worthy of note because it suggests that the cathode rays are a fundamental entity, not a part of the element from which they are emitted.

It is important to realize how difficult these questions were at the time and how experimental evidence was used to answer them. Crookes suggested that molecules in the gas collided with the cathode, somehow acquired a negative charge (i.e., became negative ions), and then “rebounded” with great speed as they were repelled by the negative cathode. These “charged molecules” would travel in a straight line, be deflected by a magnetic field, and cause the tube to glow where they struck the glass.

However, Crookes’s hypothesis was immediately attacked. Critics noted that the cathode rays could travel the length of a 90-cm-long tube with no discernible deviation from a straight line. But the mean free path for molecules, due to collisions with other molecules, is only about 6 mm at the pressure in Crookes’s tubes. There was no chance at all that molecules could travel in a straight line for 150 times their mean free path!

But if cathode rays were not particles, what were they? An alternative theory was that the cathode rays were electromagnetic waves. After all, light travels in straight lines, casts shadows, and can, under the right circumstances, cause materials to fluoresce. It was known that hot metals emit light—incandescence—so it seemed plausible that the cathode could be emitting waves. The major obstacle for the wave theory was the deflection of cathode rays by a magnetic field. But the theory of electromagnetic waves was quite new at the time, and many characteristics of these waves were still unknown. Visible light was not deflected by a magnetic field, but perhaps some other form of electromagnetic waves might be so influenced.

The controversy over particles versus waves was intense. British scientists generally favored particles, but their continental counterparts preferred waves. Such controversies are an integral part of science, for they stimulate the best minds to come forward with new ideas and new experiments.