Thanks to Prof. Everett Lipman for the idea for this demonstration.
Flip the switch on the CRT power supply from “Standby” to “On.” A green spot should appear in the center of the screen. (If it does not appear, touch the pins on the double banana plug; this will discharge the deflection plates, and the spot will appear.) Now turn on the power to the coil. If the coil is oriented and connected exactly as shown, the spot now moves upwards, the distance depending on how much current is flowing through the coil, and on the distance of the coil from the CRT. You can move the coil around to change the direction in which the spot moves, and vary the current through the coil, and its distance from the CRT, to change the distance over which the spot moves.
The earliest cathode ray tube, or CRT, enabled J.J. Thomson to establish the identity of the electron as a fundamental particle, and to determine its charge-to-mass ratio, e/m. (In a footnote to the section about the discovery of the electron, in Physics, Part Two, Robert Resnick and David Halliday write that evidence exists that a German physicist named Weichert discovered the electron several months before Thomson.) A cathode ray tube is an evacuated glass envelope having a cathode at the back, and an anode some distance in front of it. A filament near the cathode heats it, “boiling” electrons off it. A potential placed between the cathode and the anode accelerates these electrons through the tube, at the front face of which they strike a phosphor screen. Where the electrons hit the screen, they excite the phosphor, causing it to fluoresce and produce a (bright) spot on the screen. Pairs of plates, placed at right angles to each other between the anode and the screen, allow one to deflect the electron beam, or “cathode ray,” up, down, left or right as desired. By using a pair of deflection plates, measuring the beam deflection for a particular deflection potential, and then applying a uniform magnetic field to cancel the deflection, Thomson was able to calculate e/m for the electron. (It is -1.75882001076 × 1011 C/kg. Its charge is -1.602176634 × 10-19 C, and its mass is 9.1093837015 × 10-31 kg.*)
Not only was the cathode ray tube a useful research tool by itself, but in suitably modified and refined form, it was the heart of the oscilloscope (first known as the cathode ray oscilloscope), the television set and displays for computers and video equipment. While many such devices still contain cathode ray tubes, most now use liquid crystal or LED flat-panel displays instead.
Both this demonstration and the previous one in the catalogue (68.33 -- Cathode ray tube, magnet) show the deflection of an electron beam by a magnetic field. In demonstration 68.33, a bar magnet provides the magnetic field. In this demonstration, a current flowing through a coil generates the magnetic field. In both of these demonstrations, there is no potential applied to either pair of deflection plates in the CRT, so without an external magnetic field, the electron beam is undeflected and hits the center of the screen.
The force on a charged particle moving through a magnetic field is F = qv × B. If a test charge, q, moves with a velocity v through a region where a magnetic field is present, and it experiences a force F, we can define the magnetic field, B, as the vector that satisfies this equation. From this equation, we can see that the units of B are newton/(coulomb(meter/second)), or newton/(ampere·meter). This combination of units is called the tesla. The magnitude of the force is F = qvB sin θ. In the photograph above, the electrons are, of course, coming towards the front of the table. The magnetic dipole, μ, associated with an electric current flowing in a coil is μ = NiA, where N is the number of turns in the coil, i is the current (in amperes) and A is the cross-sectional area of the coil. For points along the axis of the coil, at a distance much greater than the radius of the coil, the magnitude of the magnetic field is B = (μ0/2π)(μ/x3), where μ0, the permeability constant, equals 4π × 10-7 tesla·meter/ampere. Its direction is given by the right-hand rule. If the current flow in the coil in the photograph is upwards in front and downwards in back, or clockwise if we face the right side of the coil, then the north pole is to the left, and the south pole is to the right, and the field lines at the CRT point to the left. (See the explanation for 68.13 -- Right-hand rule model.) With the magnetic field oriented this way, since q is negative, F points upward, which, as we can see from the spot on the CRT screen, is the direction in which the beam is being deflected.
In the apparatus used in this demonstration, the voltages on the cathode and the anode are, respectively, approximately -110 and +350 volts, so the electrons acquire a total kinetic energy of about 460 electron-volts by the time they exit the electron gun of the CRT. In joules, this energy is (460 eV)(1.602 × 10-19 J/eV) = 7.37 × 10-17 joules. Kinetic energy, K, equals (1/2)mv2, so v = √(2K/m), and v = √(2(7.37 × 10-17 J)/9.11 × 10-31 kg), or 1.27 ×107 m/s.
Measurement with a Hall probe shows that with the coil oriented as in the photograph and current adjusted so that the beam is deflected to the top of the screen, the magnetic field at the center of the face of the CRT is about 2.0 gauss, or 2.0 × 10-4 tesla. So the force that an electron experiences on its way to the screen of the CRT is on the order of (1.602 × 10-19 C)(1.27 × 107 m/s)(2.0 × 10-4 T) = 4.1 × 10-16 N.
You can also hold the coil edge-on, that is, with the plane of the coil intersecting the axis of the CRT. This is equivalent to showing the deflection caused by the electric field around a bundle of parallel wires. With the coil held horizontally, the field lines at the CRT are vertical, and the beam is deflected horizontally. Varying either the distance between the coil and the CRT, or the current flowing through the coil (by adjusting the voltage on the power supply), changes the magnitude of the beam deflection. Flipping the orientation of the coil 180° or reversing the current, of course, reverses the direction of deflection.
*These values are from the Committee on Data for Science and Technology (CODATA) 2018 data set.
References:
1) Halliday, David and Resnick, Robert. Physics, Part Two, Third Edition (New York: John Wiley and Sons, 1977), pp. 718-21, 727, 736-7, 761.