![]() Since Thomson knew both the electrical and magnetic field strengths as well as the amount of deflection, he could easily solve for the velocity. Then, the mass, charge and both lengths cancel, leaving us with: ![]() ![]() (Several years previous to 1897, Thomson had measured the cathode rays' velocity, but he grew to distrust the results.) However, both equations can be used as a ratio if the deflections by the two fields are made to be equal. Notice that both equations depend on knowing the velocity of the cathode ray. A different experiment would have to be carried out. However, the two formulas above could not give either the charge or the mass by itself. Just below are GIFs of each formula.īy carrying out the experiments and measuring the proper values, he could calculate what the charge-to-mass (e/m) ratio was for the cathode ray. Thomson had developed formulas based on the deflection of the cathode ray by the electric field and by the magnetic field. (May 1996: This will be made a link when that section is written.)įor a fuller discussion of the below, please see "The Discovery of Subatomic Particles" by Steven Weinberg. Knowledge of the value of 'e' or of 'm' would be needed to get to the other once you knew e/m, which Thomson did know.Įlsewhere you will find discussion of how the value for 'e,' the charge on the electron was determined. The e/m ratio is important because that is as far as Thomson could get with his cathode ray tubes. Reversing the above figures and using grams rather than kilograms gives a value of 5.686 x 10 -9 g/C. Many textbooks and articles use the m/e ratio, that is the mass-to-charge ratio. Often, books round off the 1.759 portion to 1.76. Usually, grams are used rather than kilograms giving a numerical value of 1.759 x 10 8. Therefore, the modern value for the e/m ratio is 1.759 x 10 11 C/kg. The modern value for the charge on the electron (to four significant places) is 1.602 x 10 -19 coulombs and the electrons mass is 9.109 x 10 -31 kilograms. ![]() The cathode rays bend toward the positive pole, confirming that cathode rays is negatively charged.Į/m ratio stands for charge-to-mass ratio of the electron. Thomson is the first individual to succeed in deflecting the cathode ray with an electrical field. The discovery of this effect in 1855 predates by some ten years the unification of electricity and magnetism by James Clerk Maxwell.Ĥ) Although there was some speculation that the cathode rays were negatively charged, it is not shown to be true by experiment until 1895, just two years before Thomson announces the electron.ĥ) J.J. This showed that the cathode ray carried energy and could do work.ģ) The cathode ray is deflected from a straight line path by a magnetic field, suggesting that the two were related in some way. This showed that the cathode rays traveled in straight lines.Ģ) The cathode ray can push a small paddle wheel up an incline, against the force of gravity. Pierre Curie carried a vial of radium in his coat pocket to demonstrate its greenish glow, a habit that caused him to become ill from radiation poisoning well before he was run over by a horse-drawn wagon and killed instantly in 1906.There were a number of results gathered over the years by cathode ray tube researchers.ġ) If an object is placed in the path of the cathode ray, a shadow of the object is cast on the glowing tube wall at the end. Starting with several tons of pitchblende, the Curies isolated two new radioactive elements after months of work: polonium, which was named for Marie’s native Poland, and radium, which was named for its intense radioactivity. She found that one particular uranium ore, pitchblende, was substantially more radioactive than most, which suggested that it contained one or more highly radioactive impurities. Marie Curie coined the term radioactivity (from the Latin radius, meaning “ray”) to describe the emission of energy rays by matter. Becquerel’s work was greatly extended by Marie Curie (1867–1934) and her husband, Pierre (1854–1906) all three shared the Nobel Prize in Physics in 1903. The second line of investigation began in 1896, when the French physicist Henri Becquerel (1852–1908) discovered that certain minerals, such as uranium salts, emitted a new form of energy. With this information and Thomson’s mass-to-charge ratio, Millikan determined the mass of an electron: Subsequently, the American scientist Robert Millikan (1868–1953) carried out a series of experiments using electrically charged oil droplets, which allowed him to calculate the charge on a single electron. Another set of electrode plates deflect the ray, with the ray bending towards the positive plate. Schematic of cathode ray tube with deflection. Image used with Permission (CC BY-SA-NC). As the cathode rays travel toward the right, they are deflected toward the positive electrode (+), demonstrating that they are negatively charged. \): Deflection of Cathode Rays by an Electric Field.
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