His second experiment focused on the effect of an electric field on the rays. Thus Thomson showed that the rays and electric current were connected. When he deflected the cathode rays away from the electrometer, the current was also deflected. ![]() Thomson constructed a cathode ray tube with an electrometer which could detect the current within the tube. It was not clear, however, if this deflection of the rays also deflected the electric current. It was known that when a magnet is placed near a cathode ray tube, the rays will be deflected by the magnet. The first experiment was to determine if the rays had an electric charge. Popular Science Monthly (1903) Schematic showing cathode ray deflection by a magnet. To demonstrate this he conducted three main experiments. Thomson thought they might be a component of atoms themselves. Various experiments had been performed with cathode ray tubes, but the results didn’t seem to point to a clear solution. There was an indication from chemical experiments that materials were made of elemental atoms, but how they interacted, or what might compose them, was still quite speculative. Thomson undertook three experiments that changed our understanding of atomic theory.Īt the time, atomic theory was still in its infancy. There were lots of ideas proposed to explain cathode rays, but none were particularly satisfactory. ![]() By extension, the glass tube became known as a cathode ray tube. By evacuating more air out of the glass tube, it soon became clear that the light was emitting from the cathode, and so they were called cathode rays. It was clear that an electric current ran from one wire (the cathode) to the other (anode) through the tube, but it was not clear what was causing the glow. When you did this, the tube would glow, somewhat like a neon light. Take a partially evacuated glass tube with wires embedded on either end and run a high voltage across it. In the late 1800s there was an interesting physics demonstration that became rather popular. Science Museum London, CC BY-SA J J Thomson’s 1897 cathode ray tube with magnet coils – used to discover the electron.
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