# Lab 3 - Measuring the Mass-Charge Ratio of Cathode Rays...

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Measuring the Mass-Charge Ratio of Cathode Rays Eric Tao, Andrew Dewhurst February 11st, 2013 Abstract Cathode rays have been observed and studied since antiquity, and here we attempt to quantify the ratio of a cathode ray’s charge with respect to its mass. By using a set starting energy and then balancing the effects of an electrical field acting on the cathode ray against a magnetic field, we were able to determine both the velocity of the cathode ray particles as well as their ratio of charge to mass. 1 Introduction Since the 18th century, people have studied the behavior electricity, especially in low pressure situations. As the technology was more refined, a clear ”beam” started to form, emanating from the cathode and traveling towards the anode. The next logical step was then to test its properties. Through experimentation, it was found that these beams were affected by both electrical and magnetic fields, consistent with that of charged particles. However, measuring the charge to mass ratio of these particles would constitute an insight into their essential character. Using an apparatus that can apply both an electrical field and a magnetic field, we will first observe the qualitative effects of the fields on the beam and then attempt our charge-mass analysis. We note that with the electrical field, the cathode ray was deflected downwards, telling us that the electrical field was pointing upwards, and that the cathode ray has a negative charge associated with it. It leaves the beam as a mostly coherent path, deflecting the particles fairly evenly. A stronger electrical field increases the deflection, while a greater beam voltage decreases the deflection. In addition, it only deflects

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