SE-9629
Electron Charge-to-Mass Ratio
10
012-14265A
•
Current Adjust: Sets the current through the Helmholtz coils.
•
Output: Output power.
•
Current Display Window: Displays the current through the Helmholtz coils.
Experiment Procedure
Adjust Operating Voltages and Current
•
To get a clearer view of the electron beam, conduct the experiment in a darkened room.
1. Connect all the cables and cords as described previously.
2. On the Tunable DC (Constant Voltage) Power Supply II, set the Voltage Range Switch to 0 – 200 V
(
)
.
3. For both power supplies, push in the Power Switch to the ON position.
4. On the Tunable DC (Constant Voltage) Power Supply II, set the Accelerating Voltage (anode) to 120 V DC.
5. Wait several minutes for the filament to heat up*. When it does, you will see the elec-
tron beam emerge from the electron “gun”. The electron beam is initially horizontal
and is visible as a dim, bluish ray.
6. On Power Supply II, adjust the voltage output to the Accelerating Voltage to optimize
the focus and brightness of the electron beam.
7. On the Tunable DC (Constant Current) Power Supply,
3.5 A / 6.3 V
, increase the cur-
rent to the Helmholtz coils. Watch the electron beam and check that the electron beam
curves upward.
•
If the electron beam is not deflected at all, reverse the polarity of one of the Helmholtz
coils so that current passes through both coils in the same direction.
•
If the electron beam does not curve upward, swap the connections on the
3.5 A
output terminals on the Power Supply.
8. Continue increasing the current until the electron beam forms a closed circle.
•
If the electron beam does not form a closed circle, slightly rotate the Platform to the right or left to align the magnetic field
generated by the Helmholtz coils with the magnetic field of Earth.
Record Data: Standalone
9. Carefully read the Current Display to find the current (
I
H
) through the Helmholtz coils and record the value in Table 1.
Carefully read the Voltmeter and record the Acceleration Voltage (
U
) in Table 1.
10. Carefully measure the radius,
r
, of the electron beam. Look through the e/m tube at the Mirrored Scale. To avoid parallax
errors, move your head to align the electron beam in the tube with the reflection of the beam as you see it in the Mirrored
Scale. Measure the radius of the electron beam as you see it on both sides of the Mirrored Scale, and then average the
results. Record the average radius in Table 1.
11. Record other series of measured values for different Accelerating Voltages (
U
) and current (
I
H
) through the Helmholtz
coils. Record your measurements in Table 1.
Note:
Before switching on the power, be sure that all voltage and current
controls are turned fully counterclockwise.
*
NOTE
: It is very important
to allow the e/m tube and
apparatus to warm up for
several minutes prior to
making any measurements.
