Purpose: To show the path of electrons in a magnetic field and to find the charge-to-mass ratio.

Turn on the Heathkit power supply, which provides filament current and accelerating voltage for the electron tube. The filament current should be set to 6.3 A, and the accelerating voltage ("B⁺" on supply) between 150 and 300 V -- about 200 V seems to work best. When the tube filament warms up so that the electron beam appears, explain why the beam shows the path of the electrons. The electron beam can be focused by means of the "focus" control on the e/m apparatus. Use the bar magnet to show qualitatively the effect of a field on the electron path.

Turn on the Southwestern Technical power supply that supplies current to the Helmholtz coils. Leave the controls of the power supply fixed, and vary the current to the coils by means of the current control knob on the e/m apparatus.

If you want to calculate e/m, you can read the path radius by using the calibrated mirror. This mirror attaches to the housing of one of the Helmholtz coils. (Note: The Southwestern Technical power supply tends to overheat, and should not be left on for extended periods of time.)

Use a close-up camera so that the class can see the electron beam.

References:

Manual: None