Category: Atoms And Radioactivity

The tube that Thomson built for his electrostatic deflection experiment (Figure 46) is the grandfather of the modern oscilloscope CRT.** As shown in Figure 48, the basic structures of Thomson’s tube are still recognizable in the modern CRT. The main difference is in the replacement of the “cold cathode,” where cathode-ray production relies on stripping electrons by… Continue reading THOMSON AND THE MODERN CRT

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… Continue reading THOMSON’S SECOND EXPERIMENT—ELECTROSTATIC DEFLECTION OF CATHODE RAYS

Hertz’ (mistaken) finding that cathode rays were not deflected by an electric field was a true mystery, given that cathode rays were so easily bent by magnetic fields. In 1895, French physicist, and later Nobel Prize laureate, Jean Perrin built a CRT to investigate whether or not cathode rays transported charge.10 A diagram of Perrin’s original… Continue reading THOMSON’S FIRST 1897 EXPERIMENT—NEGATIVE CHARGE AND RAYS ARE JOINED TOGETHER

In 1879, Crookes had reached the conclusion that cathode rays must be particles of some sort, observing that they traveled in straight lines and were stopped by metallic objects in their path. As a demonstration, Crookes inserted an electrode shaped like a Maltese cross in the tube, and it cast a sharp shadow on the… Continue reading CATHODE-RAY TUBES

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… Continue reading THE DISCOVERY OF THE ELECTRON

Our CRTs require a beam of electrons, which we produce using the assembly shown in Figure 41. Electrons are stripped off the cathode by a process known as glow discharge between an aluminum cathode rod and a hollow anode. The electron gun is built inside an Ace-Thred #11 connector (Ace catalog number 7644-10). The cathode is a 3/8-in. aluminum… Continue reading THE ELECTRON GUN

Electron beams in a good vacuum are invisible, but a screen coated with a material that fluoresces when hit by electrons can be used to make the beam visible. These fluorescent materials are called phosphors, and are used most commonly in cathode-ray oscilloscope and TV screens to produce an image by steering an electron beam inside… Continue reading PHOSPHOR SCREENS

Making vacuum tubes from scratch is almost a lost art. It involves plenty of practice in technical glassblowing, as well as an understanding of materials and vacuum techniques. However, vacuum tubes that operate at relatively high pressures and require continuous pumping are easy to build by cobbling together glass tubes, copper piping, and rubber corks.… Continue reading A VACUUM TUBE LEGO^® SET

We will also require us to use very high voltages—well in excess of the 2,000 V that can be obtained from the power supplies we built to power the PMT probe. Fortunately, precise regulation at these voltages is not needed, so a high-voltage power supply that can produce over 100,000 V is easy to build. Figure… Continue reading A VERY-HIGH-VOLTAGE POWER SUPPLY

Simple mechanical pressure gauges don’t usually work at pressures below 10 Torr, so we need a different way of measuring pressure in our vacuum systems. We use the most inexpensive pressure sensor that works at these levels, which is the T/C gauge. It operates by measuring the thermal conductivity of the gas inside the chamber. As… Continue reading THE VACUUM GAUGE