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Charge to Mass Ratio

Procedure

4.1 Charge-to-Mass Ratio

WARNING: increasing the filament current too quickly can permanently damage the electron source. If you are using the older apparatus, turn the current knob on the filament supply slowly until you see the current in the milli-amp detector start to increase. If you are using the newer power supplies, make sure that you are adjusting the current by 0.1 A at a time or less. DO NOT CHANGE THE CURRENT BY 1 AMP AT A TIME. DO NOT GO OVER THE INDICATED LIMITS ON THE MILLIAMP DETECTORS. DO NOT ADJUST THE VOLTAGE OF THE FILAMENT POWER SUPPLIES.

  1. The electrical circuit is already connected as shown in Figure 2. Study the apparatus and be sure you can locate each component in the diagram and on the apparatus.
  2. Check the compass on the table, and check if the apparatus is in the magnetic north-south direction. Determine the dip angle using the dip needle. Record the value of the dip angle and set the apparatus to the complement of this angle. The magnetic field of the Helmholtz coils is now aligned with that of the earth.
  3. Be certain to follow the warnings in Section 3.1 before continuing.
  4. Before turning on the filament, be certain that a potential of at least 20 volts is applied to the cylindrical anode; do not increase the anode current above 8 mA.
  5. Increase the filament current until you see a bluish beam exiting the side of the cylindrical anode. Now send a small current through the Helmholtz coils. The magnetic field of the coils will straighten the beam or curve it in the opposite direction.
  6. Determine the magnet current at which the electron beam trajectory is a straight line. The magnetic field at this current exactly cancels the earth’s field. All magnet currents should be made relative to this value.
  7. Increase the field current until the electron beam describes a circle. Record the current at which the outer edge of the electron beam hits the farthest edge of the various crossbars. Compare your data to the expected trend.
  8. Take the same data set as above with several values of accelerating voltage between 20 V and 50 V.
  9. Increase the magnet current so that the electron beam trajectory is a fairly small circle. Carefully rotate the apparatus so that the field from the Helmholtz coil is no longer aligned with earth’s magnetic field. Note the change in the electron beam trajectory.

4.2 Precision Measurement

The multimeters you’ve been using to measure current and voltage are only accurate to about 1%. We can increase that precision by using better equipment: a shunt resistor and precision digital multimeter

  1. Disconnect the ammeter from the Helmholtz coil circuit. Connect the shunt resistor in series with the Helmholtz coil, and connect the digital multimeter in parallel across the shunt resistor in order to measure the voltage across it. The resistance of the shunt resistor is 0.01\Omega ± 0.1%
  2. Turn on the digital multimeter and make sure that you are measuring in mV (the most precise setting it has). Turn up the current from the power supply to get the electron beam to hit the first distance- measurement pin. Do a quick check that Ohm’s law still holds.
  3. The shunt resistor/power supply may need 10-15 minutes to warm up. If you notice the measured voltage drifting in a single direction, then you may need to wait a few minutes.
  4. Repeat your measurement of the charge to mass ratio for a single accelerating voltage using your new precision measurements of the Helmholtz coil current. Try to be as careful as possible to make sure that the electrons that experience the full accelerating potential are hitting the measurement pin. Make these measurements as accurately and as precisely as possible. Take your time.

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Physics 2P03 Lab Manual Copyright © by Daniel FitzGreen. All Rights Reserved.

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