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CHEMISTRYTopic: Atomic Structure - (Cathode Rays) By Kingsley Idiagbor, B.Sc (Hons), PGDCsNCE Early last century at about 1906,a host of scientists led by Sir William Crooke challenged the Dalton's Atomic Theory. To drive home their objection, they used a discharged tube with two electrodes at either end. The discharge tube is evacuated of air while electric voltage was passed through it. Subsequently, the envelope of the entire tube fluoresced (or glowed). It was noticed that the glow was caused by rays emanating from the cathode. The rays were therefore called cathode rays. J.J. ThompsonJ.J.Thompson (1909) carried out experiments in order to investigate the nature and the characteristics of the cathode rays. , he blocked the path of the cathode rays with an object (a Maltese cross) in a modified discharge tube called cathode ray tube. The shadow of the Maltese cross object was cast on the fluorescent screen of the cathode ray tube (See Fig.1) Deduction: The inference from this experiment is that the cathode rays, just like light, travel in straight line. Secondly, Thompson subjected the cathode rays to magnetic and electric fields. He observed that the cathode rays were deflected by the magnetic field and moved towards the positive electric plate (see Fig.2) Deduction: The clear deduction from the experiment is that cathode rays must consist of negatively charged particles. In the third experiment, J.J.Thompson placed tiny paddle wheels along the path of the cathode rays. This resulted in the gentle spinning of the tiny paddle wheels by |