SE-9629
Electron Charge-to-Mass Ratio
12
012-14265A
1. Setup the equipment as described above for measuring e/m except do not supply current to the Helmholtz coils.
2. Apply 6.3 V AC to the “Filament” terminals and 120-200 V DC to the terminals of the “Accelerating Voltage”. Wait sev-
eral minutes to warm up the cathode.
3. When the electron beam appears, connect a 0-100 V DC power supply to the terminals labeled “Deflection Plates (Upper
and Lower)”. Slowly increase the voltage to the deflection plates from 0 V to approximately 100 V DC.
Note the deflection of the electron beam. Note the beam direction.
Demonstrations
1. Instead of using the Helmholtz coils to bend the electron beam, you can use a permanent magnet to show the effect of a
magnetic field on the electron beam. Just provide the following power to the e/m apparatus:
Filament: 6.3 V AC
Accelerating Voltage: 120 - 200 V DC
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When the electron beam appears, use your permanent magnet to bend the beam.
2. The socket for the e/m tube is designed so that the tube can be rotated. The tube can therefore be oriented so it is at a cer-
tain angle, with respect to the magnetic field from the Helmholtz coils. By setting up the equipment as for measuring e/m,
you can rotate the tube and study how the beam deflection is affected.
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Figure 1: The electron beam is horizontal, dim, and bluish.
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Figure 2: As current through the coils increases, the electron beam curves to form a closed circle.
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Figure 3: Turning the e/m tube and platform can cause the electron beam to form a spiral path.
3. With no magnet and no current in the Helmholtz coils, rotate the tube or the entire apparatus to see the deflection of the
beam due to the Earth’s magnetic field. Is the direction of the deflection of the beam as you expect?
