Shortly after Röntgen’s discovery of x rays, the young English physicist J. J. Thomson began using them to study electrical conduction in gases. He found that x rays could discharge an electroscope and concluded that they must be ionizing the air molecules, thereby making the air conductive.

This simple observation was of profound significance. Until then, the only form of ionization known was the creation of positive and negative ions in solutions where, for example, a molecule such as NaCl splits into two smaller charged pieces. Although the underlying process was not yet understood, the fact that two atoms could acquire charge as a molecule splits apart did not jeopardize the idea that the atoms themselves were indivisible. But after observing that even monatomic gases, such as helium, could be ionized by x rays, Thomson realized that the atom itself must have charged constituents that could be separated! This was the first direct evidence that the atom is a complex structure, not a fundamental, indivisible unit of matter.

Thomson was also investigating the nature of cathode rays. Other scientists, using a Crookes tube like the one shown in FIGURE 37.11a, had measured an electric current in a cathode-ray beam. Although its presence seemed to demonstrate that the rays are charged particles, proponents of the wave model argued that the current might be a separate, independent event that just happened to be following the same straight line as the cathode rays.

FIGURE 37.11 Experiments to measure the electric current in a cathode-ray tube.

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Thomson realized that he could use magnetic deflection of the cathode rays to settle the issue. He built a modified tube, shown in FIGURE 37.11b, in which the collecting electrode was off to the side. With no magnetic field, the cathode rays struck the center of the tube face and created a greenish spot on the glass. No current was measured under these circumstances. Thomson then placed the tube in a magnetic field to deflect the cathode rays to the side. He could determine their trajectory by the location of the green spot as it moved across the face of the tube. Just at the point when the field was strong enough to deflect the cathode rays onto the electrode, a current was detected! At an even stronger field, when the cathode rays were deflected completely to the other side of the electrode, the current ceased.

This was the first conclusive demonstration that cathode rays really are negatively charged particles. But why were they not deflected by an electric field? Thomson’s experience with the x-ray ionization of gases soon led him to recognize that the rapidly moving cathode-ray particles must be colliding with the few remaining gas molecules in the tube with sufficient energy to ionize them. The electric field created by these charges neutralized the field of the electrodes, hence there was no deflection.

Fortunately, vacuum technology was getting ever better. By using the most sophisticated techniques of his day, Thomson was able to lower the pressure to a point where ionization of the gas was not a problem. Then, just as he had expected, the cathode rays were deflected by an electric field!

Thomson’s experiment was a decisive victory for the charged-particle model, but it still did not indicate anything about the nature of the particles. What were they?