THOMSON’S FIRST 1897 EXPERIMENT—NEGATIVE CHARGE AND RAYS ARE JOINED TOGETHER

Hertz’ (mistaken) finding that cathode rays were not deflected by an electric field was a true mystery, given that cathode rays were so easily bent by magnetic fields. In 1895, French physicist, and later Nobel Prize laureate, Jean Perrin built a CRT to investigate whether or not cathode rays transported charge.¹⁰ A diagram of Perrin’s original tube is shown in Figure 44a. His idea was to have cathode rays flow between the cathode and anode electrodes. However, a hole in the anode would allow some of the cathode rays to go into the hollow anode tube, where they would be probed by a collecting electrode. The very fine gold leaves on an electrometer would spread apart if the cathode rays conveyed charge to the collection electrode. This is because the leaves repel each other when they acquire similar electric charges. Their separation is a direct indication of the net charge stored on them. In addition, Perrin hypothesized that the collected charge should disappear as soon as the source of the radiation was removed if the cathode rays were some form of electromagnetic radiation. The fact that charge remained on the electroscope supported his hypothesis that cathode rays were made of electrically charged matter.

Figure 44 In 1895, French physicist Jean Perrin set out to investigate whether or not cathode rays transported charge. His CRT allowed some of the cathode rays to go into the hollow anode tube, where they would be probed by a collecting electrode.^¶ The very thin leaves on an electrometer would spread apart if the cathode rays conveyed charge to the collection electrode.

Perrin’s experiment had shown unequivocally that cathode rays transported negative charge. However, not everyone was convinced that this was not an incidental effect of the radiation that produced fluorescence in screens. In his first 1897 critical experiment toward the discovery of the electron, British physicist J. J. Thomson built a modified version of Perrin’s 1895 tube, as shown in Figure 45a. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays. Without a magnetic field present, the cathode rays followed a straight line and produced fluorescence in the glass just in front of the anode. As expected, no charge was detected by the electrometer connected to a collection electrode tilted away from the ray’s path. When a magnet was placed close to the tube, the cathode rays were bent toward the collection electrode, causing the electrometer to indicate charge. Critically however, the site of fluorescence also deviated as the magnet was brought close to the apparatus, until the fluorescence disappeared at the collector electrode. As Thomson saw it, the negative charge and the cathode rays must somehow be connected.

Figure 45 In 1897, British physicist J. J. Thomson conducted the first of three critical experiments that led him to the discovery of the electron. (a) He used a modified version of Perrin’s tube to determine if the charge and radiation elements of cathode rays were different aspects of the phenomenon. As expected, the undeflected ray produced fluorescence and no charge detected by the electrometer. When a magnet was placed close to the tube, rays were bent toward the collection electrode, causing the electrometer to indicate charge. Critically however, the site of fluorescence also deviated as the magnet was brought close to the apparatus until the fluorescence disappeared at the collector electrode. We adapted this figure from Thomson’s original 1897 paper on cathode rays.¹¹ (b) Our version of the experiment uses a digital voltmeter to detect charge deposited onto the collecting electrode.

For our version of Thomson’s tube, we simply retracted the “happy face” electrode on the side port of the flask of Figure 43 so that it would lie flat against the flask wall, as shown in Figure 45b. In addition, we connected a digital multimeter with 10 Mω input impedance between the happy face electrode’s post and ground. We set the multimeter to measure volts DC, essentially turning it into a sensitive microammeter in which the current in microamperes is equal to the voltmeter’s reading divided by 10.

When you conduct the experiment, first evacuate the tube to approximately 100 mTorr and then turn on the high voltage. As you continue pumping, the cathode rays should produce a clearly visible mark on the screen at a spot across from the exit hole of the electron gun’s beam. Stabilize the pressure and voltage to maintain that spot on the screen. Measure the voltage on the digital multimeter. Now bring a magnet close to the tube, such that the cathode rays are bent toward the happy face electrode. This task will be easy if you followed our suggestion of painting a 1-cm fluorescent stripe on the side wall of the flask between the screen and the side port. The reading on the multimeter should increase quite dramatically when the cathode rays are deflected to fall on the collection electrode. Your voltmeter will let you determine the charge conveyed by the cathode rays (try inverting the connections to the multimeter), as well as the amount of charge arriving at the collection electrode (remember that I[A] = Q[Coulomb]/s).