THOMSON’S SECOND EXPERIMENT—ELECTROSTATIC DEFLECTION OF CATHODE RAYS

For his second crucial experiment, Thomson reattempted to deflect the cathode rays, much like Hertz had unsuccessfully tried before, but this time pulling a deeper vacuum in the tube. As shown in Figure 46, the cathode rays in his tube were made to pass between two parallel aluminum plates. Contrary to Hertz’s experience, the cathode rays indeed deflected when a voltage was placed across the plates. Thomson showed that Hertz was unable to detect the deflection in his tube because the vacuum was not sufficiently deep.^|| However, the deflection could be easily detected in Thomson’s tube, even in a weak electric field, provided the vacuum was good enough. Moreover, Thomson found that the deflection was proportional to the potential difference between the plates.

Figure 46 For his second crucial experiment of 1897, Thomson built a CRT that he evacuated very deeply. The cathode rays in his tube traveled between two parallel aluminum plates. As expected, the beam would hit the fluorescent screen with no deviation (a) when the plates were not electrostatically charged. However, applying a voltage between the plates deflected the beam (b) in an amount proportional to the potential difference between the plates. We adapted this figure from Thomson’s original 1897 paper on cathode rays.¹¹

The tube pressure needed to observe electrostatic deflection is well within the capabilities of our humble refrigeration-service pump. In fact, we even need to leak a bit of air on purpose in order to keep the pressure at around 33 mTorr to keep the glow discharge going.

As shown in Figure 47, we added electrostatic deflection plates to our homemade CRT by introducing an Ace Glass segment with two sets of #7 ports placed at 90° to each other. We cut deflection plates from a thin brass sheet (purchased at a hobby store) on which we soldered a small brass screw. We keep the plates in place and bring out electrical connections through internally threaded 1/4-in.-diameter × 2-in.-long stainless steel spacers. A rubber O-ring and a #7 Ace-Thred bushing seal each cylindrical spacer against its port. To deflect the beam, we connected one of the plates in the vertical pair to ground (the vacuum pump manifold). We also connected the ground terminal of our PMT bias power supply to the same point. We connected the other plate to the high-voltage terminal of the PMT power supply, and caused a deflection of the beam in the direction of the positively charged plate by increasing the voltage of the PMT power supply. When you conduct this experiment, try correlating the amount of deflection that you see on the screen against the voltage between the plates. Do your results agree with Thomson’s observation that the cathode-ray beam deflection is proportional to the potential difference between the plates?

Figure 47 Our version of the CRT with which Thomson demonstrated that cathode rays are deflected by an electrostatic field. (a) We used our homemade CRT of Figure 40 and added an Ace-Thred #25 12-in. column (Ace Glass #7488-24 air-sampling manifold) that was custom-ordered to have six Ace-Thred #7 ports. (b) The deflection plates are supported by ¹/₄-in. diameter × 2-in. long threaded spacers. (c) We cut the deflection plates from a thin sheet of brass and soldered brass screws to each.