The charge to mass ratio of electrons was shown to be 1800 times greater than that of hydrogen ions. As a result, Thomson's experiment led to the discovery of the first subatomic particle – the electron. In addition, the constancy of the cathode ray's charge to mass ratio shows that these negatively charged particles are present in all matter. Since an actual charge to mass ratio value was determined for cathode rays, Thomson proved that cathode rays do indeed have mass, and hence are negatively charged particles. He showed that the value of charge to mass ratio remained constant. Thomson replicated the experiment using cathodes made from different metals and under various conditions. Since the value of E, B and r were known to Thomson, the value of the charge to mass ratio of the cathode ray was calculated. (We recommend understanding the derivation of the above equation to be able to provide mathematical support in exam responses) The centripetal force was provided by the force due to the magnetic field: Here, the radius of the cathode's curved path can be analysed by considering centripetal force. Under the influence of the magnetic field only, the cathode ray was deflected downwards. When this occurred, the force acting on the cathode ray due to the electric field balanced the force due to the magnetic field.Īfter a straight trajectory was achieved, the electric field was switched off. Thomson adjusted the strength of the magnetic field until the cathode travelled a straight path. The direction of this magnetic field was perpendicular to the electric field, and positioned such that it would cause the cathode ray to deflect downwards. Next, Thomson applied a uniform magnetic field using current-carrying coils.
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