However, other researchers, notably Crookes, argued that the focused nature of the beam meant that they had to be particles. With more experimentation, researchers found that the ‘cathode rays’ emitted from the cathode could not move around solid objects and so traveled in straight lines, a property of waves. William Crookes discovered that a tube coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when rays from the electron gun hit it. Later and improved cathode ray experiments found that certain types of glass produced a fluorescent glow at the positive end of the tube. This cathode ray also became known as an ‘electron gun’. Physicists in the 19th century found out that if they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow. Look at any glowing neon sign or any ‘old-fashioned’ television set, and you are looking at the modern descendants of the cathode ray tube.
The image and video below show this experimental test.Even without consciously realizing it, most of us are already aware of what a cathode ray tube is. By applying a voltage between 500 and 800 V, a clear deflection of the beam may be obtained. This experimental test can also be easily carried out with our cathode tube, as it is provided with a pair of deflection plates positioned close to the beam. The force is proportional to the intensity of the electric field, and is directed in the opposite direction since the charge of the electron is negative, as shown in the picture below.
The cathode ray beam is obviously sensitive to electric fields as well. The image and video below show this experimental test. This phenomena can be easily verified by using our cathode tube and a permanent neodymium magnet: by approaching the magnet to the tube, the beam is deflected low or upward depending on the polarity of the magnet. The picture below illustrates the effect of Lorentz interaction. Given the vector product between v and B, the force is perpendicular to both the direction of the magnetic field and the velocity direction. In our case, the electric field is null and the moving charged particles are electrons, so the formula becomes: Generally speaking, the force exerted on a charge q moving with speed v within a magnetic field B and electric field E, is calculated with the following formula : These are therefore negatively charged moving particles in a straight line, as such they are subject to the Lorentz force, which governs the motion of charged particles within magnetic fields. Magnetic DeflectionĪs we know, cathode rays are made up of electrons that are accelerated from the cathode to the anode.
Thomson cathode ray experiment lorentz law generator#
The high voltage generator is a compact module easily available online, however you can use any HV generator, such as the ones based on diodes and capacitors in Cockcroft-Walton voltage multiplier configuration. The high voltage between anode and cathode is obtained with a HV generator powered with a 12 V, it generates a voltage value of 10 KV. Two plates, used to demonstrate the influence of an electric field on the electron beam, complete the fixture. It is a tube equipped with cathode and anode, with a mask with a slit so as to produce a planar cathode ray beam, there is also a fluorescent screen so as to highlight in a visual manner the presence of the beam. The picture below illustrates the operation of a Crookes tube in a schematic way.įor experiments with cathode ray tube we used an educational model readily available on eBay. The term Crookes tube is also used for the first generation, cold cathode X-ray tubes, which evolved from the experimental Crookes tubes and were used until about 1920. Wilhelm Röntgen discovered X-rays using the Crookes tube in 1895. Crookes tubes are now used only for demonstrating cathode rays. Thomson’s 1897 identification of cathode rays as negatively charged particles, which were later named electrons. It was used by Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard and others to discover the properties of cathode rays, culminating in J.J. When a high voltage is applied between the electrodes, cathode rays (electrons) are projected in straight lines from the cathode. A Crookes tube is an early experimental electrical discharge tube, with vacuum, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were discovered.ĭeveloped from the earlier Geissler tube, the Crookes tube consists of a partially evacuated glass bulb of various shapes, with two metal electrodes, the cathode and the anode, one at either end.
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