Present Occasions | by Brian Koberlein



1 Might 2014

J J Thomson’s 1897 cathode ray tube with magnet coils – used to discover the electron.
Science Museum London, CC BY-SA
J J Thomson’s 1897 cathode ray tube with magnet coils – used to find the electron.

Within the late 1800s there was an fascinating physics demonstration that turned reasonably in style. Take {a partially} evacuated glass tube with wires embedded on both finish and run a excessive voltage throughout it. Once you did this, the tube would glow, considerably like a neon mild. It was clear that an electrical present ran from one wire (the cathode) to the opposite (anode) by means of the tube, but it surely was not clear what was responsible for the glow. By evacuating extra air out of the glass tube, it quickly turned clear that the sunshine was emitting from the cathode, and they also had been referred to as cathode rays. By extension, the glass tube turned often known as a cathode ray tube. There have been a lot of concepts proposed to elucidate cathode rays, however none had been notably passable. Then J. J. Thomson undertook three experiments that modified our understanding of atomic idea.

On the time, atomic idea was nonetheless in its infancy. There was a sign from chemical experiments that supplies had been manufactured from elemental atoms, however how they interacted, or what would possibly compose them, was nonetheless fairly speculative. Numerous experiments had been carried out with cathode ray tubes, however the outcomes didn’t appear to level to a transparent answer. However J. J. Thomson thought they could be a part of atoms themselves. Some form of subatomic particle. To reveal this he carried out three principal experiments.

Schematic showing cathode ray deflection by a magnet.
Standard Science Month-to-month (1903)
Schematic exhibiting cathode ray deflection by a magnet.

The primary experiment was to find out if the rays had an electrical cost. It was identified that when a magnet is positioned close to a cathode ray tube, the rays might be deflected by the magnet. It was not clear, nonetheless, if this deflection of the rays additionally deflected the electrical present. Thomson constructed a cathode ray tube with an electrometer which may detect the present throughout the tube. When he deflected the cathode rays away from the electrometer, the present was additionally deflected. Thus Thomson confirmed that the rays and electrical present had been related.

His second experiment targeted on the impact of an electrical discipline on the rays. One of many complicated outcomes of earlier experiments was that the rays had been deflected by a magnetic discipline, however not by an electrical one. If the rays had been because of a present of charged particles, then they need to be deflected by each. Thomson famous that this isn’t solely true. If a present is surrounded by a conductor, then it gained’t be deflected by an electrical discipline. Thomson thought the gasoline within the tube could be performing as a conductor to protect the rays from an electrical discipline. So he evacuated the tube nonetheless additional, and located that in a excessive vacuum the rays had been deflected by an electrical discipline.

These two experiments satisfied Thomson that his particle concept was appropriate. So for his third experiment he rigorously measured each the power given to those particles by the electrical voltage, and the exact quantity the rays had been deflected by magnetic fields of various strengths. From this he decided the ratio of the particles’ cost to their mass. The cost to mass ratio he obtained meant that these particles had a really massive cost, or their mass was extraordinarily small. A lot smaller than the mass of hydrogen. Thomson suspected it was the latter, and subsequent experiments proved him proper.

Via his work, J. J. Thomson had demonstrated that cathode rays had been produced by negatively charged subatomic particles. We now name them electrons, and for his or her discovery Thomson was awarded the Nobel prize in Physics in 1906.

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