In this experiment we will measure the ratio of the

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Unformatted text preview: e electron are its charge e and its mass m. In this experiment we will measure the ratio of the two (e/m) with the method first used by J.J. Thomson in 1897. The experiment is based on the fact that a charged particle moving in a magnetic field feels a force at right angles to its velocity: FB = qvxB. If we send a beam of electrons into a magnetic field uniform in strength and direction, then the trajectory of the electrons is a circle whose radius depends on e/m. We measure the radius of the circle for different values of B, and deduce e/m. The Electron Beam To produce a beam of electrons, we heat a metal plate called a cathode and boil electrons off of its surface. (We won’t worry about the details of this boiling off here.) The cathode is held at a low voltage, and the boiled-off electrons accelerate towards a high-voltage plate a few centimeters away called an anode. Some electrons pass through a small hole in the anode and are collimated into a narrow beam (see figure 1). The electrons are not accelerated further once they pass through the anode. Since human eyes can’t see electrons, the whole experiment is encased in an evacuated glas...
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