Some of the most important experiments to explain radiation were conducted in 1897 by British physicist J. J. Thomson. These experiments led Thomson to the discovery of the electron, for which he received the 1906 Nobel Prize in Physics. The story starts in 1857, when German physicist and glassblower Heinrich Geissler pumped the air out of a glass tube fitted with wires at both ends. The glass tube would produce glowing streamers when high voltage was applied across the electrodes—something that Victorians found extremely interesting and entertaining. Let’s build our own tubes to see these effects firsthand, and ultimately understand the reasoning that Thomson employed in his discovery of the electron!
Our basic glow-discharge tube is shown in Figure 42a. It consists of a 30-cm-long Ace-Thred #11 glass tube (Ace catalog number 5820-04) that is terminated by a 3/8-in. aluminum rod at one end and by a 3/8-in. tube on the other end. Each of these is sealed by an Ace-Thred #11 bushing (Ace catalog number 7506-02). The tube connects directly to the vacuum manifold (we use a KF-16 to 3/8-in.-OD tube half-nipple, Kurt J. Lesker catalog number HN-SPL219). The aluminum rod is prevented from being sucked into the tube by a 3/8-in. ID shaft collar. The negative output of the high-voltage power supply of Figure 39 connects to the aluminum rod, while the ground terminal connects to the vacuum port.
Figure 42 A glow-discharge tube simply requires two electrodes sealed within an evacuated glass tube. (a) Our tube consists of a 30-cm-long Ace-Thred #11 glass tube terminated by a 3/8-in. aluminum rod at one end and by a 3/8-in. tube on the other end, which connects directly to our vacuum manifold. (b) As the pressure in the tube is lowered, many interesting features appear in the glow discharge between the electrodes.
Nothing happens at atmospheric pressure, since the voltage of the power supply is too low to bridge through approximately 25 cm of air inside the column. However, a glow discharge in the form of a thin streamer forms between the electrodes once the pump is turned on and the pressure drops to around 100 Torr (roughly one-tenth of an atmosphere). As the pressure continues to drop, the streamer becomes more vivid, radiating with a pretty pink/violet glow.
When the pressure is lowered to around 5 Torr, the pink glow pulls away from the cathode, leaving a bright bluish glow next to the cathode and a dark space (known as the Faraday space) between the pink and blue light areas. At around 1 Torr, the blue negative glow separates from the cathode. At the same time, the pink positive column breaks up into a series of separate stripes (striations). These glow phenomena captivated the attention of physicists and noblemen of the time. Geissler had developed a mercury vacuum pump capable of evacuating tubes down to around 2 Torr, making it possible for him to produce and sell his Geissler tubes to universities, as well as for home entertainment.
By 1870, British physicist Sir William Crookes was using an improved vacuum pump to draw an even better vacuum out of a Geissler tube, and noticed a fluorescent glow in the glass close to the cathode electrode. The fluorescence of the glass made him conclude that some kind of ray was being emitted by the cathode, causing the minerals in the glass to glow. Crookes also observed that the glow inside the tube didn’t start right at the cathode, but that there was a “dark space” between the metallic electrode and the glowing column. Furthermore, as he pumped more air out of the tube, the dark space grew toward the anode, until the tube was totally dark. At that point, fluorescence of the glass appeared all the way close to the anode.
As you will notice when you conduct your own experiments, the glass walls of the tube start to fluoresce at around 500 mTorr under the bombardment of rays within the tube. You can steer these rays and the fluorescence they cause to different parts of the tube’s wall by bringing a strong magnet close to the tube (just be careful of getting too close to the high-voltage cathode!).§
The nature of these rays was a mystery in the 1870s. One possibility was that they were a type of wave, perhaps one of Maxwell’s electromagnetic waves, just like light, but at a different wavelength. Another possibility was that these cathode rays were made of some kind of material particle. This latter view was supported by the fact that waving a magnet near the glass would move the rays around, causing the presumably negatively charged particles to be pushed around.
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