Figure 11-1: Schematic diagram of the e/m tube and the associated
power supplies.
The dashed gray lines indicate wiring that runs beneath the unit.
Voltages are to be set and measured as described in the experimental
procedure.
PROCEDURE
1. Open the Worksheet
and fill in the header information.
2. Record the serial number of your apparatus and its turns/coil in cells E10 and E12 on the Worksheet. Measure and record the outer and inner diameters of the Helmholtz coils and record these data in cells D15:D16.
3. Calculate the average diameter and enter the result in cell D17. Take half of this value to obtain the average radius of the Helmholtz coils and enter in cell D18.
4. With the magnetic compass, explore the space around the Helmholtz coils to see if there is a strong external magnetic field in this space. Keep the meters well away from the e/m tube. If so, move to another spot.
5. Your apparatus should already be connected as shown in Fig. 11-1. Check the circuit to make sure it is connected properly:
The filament (+ and − or cathode) is connected to the 5 V dc fixed supply.
The plate terminals are connected to the 50 V dc fixed supply.
The grid terminals are connected to the 0 to 30 V dc @ 1 A variable (with control knob) supply.
The field terminals are connected to the 0 to 30 V dc @ 5 A variable (with control knob) supply.
When you are confident that it is correct, proceed.
6. Apply power to the filament circuit. The filament current should be maintained in the filament for about 2 minutes before the high voltage is applied to the plate.
DO NOT TURN THE FILAMENT CURRENT OFF UNTIL THE GRID AND PLATE VOLTAGES HAVE BEEN REDUCED TO ZERO!
7. After the filament is heated, apply the plate voltage by connecting the lead. This voltage is set at exactly 50 volts. Record the plate voltage in cell D19.
Figure 11-1: Schematic diagram of the e/m tube and the associated
power supplies.
The dashed gray lines indicate wiring that runs beneath the unit.
Voltages are to be set and measured as described in the experimental
procedure.
8. Turn on the grid supply and adjust the grid voltage control
to get a well-defined and focused electron beam. It should be a
uniform diameter of about 2 mm and should look
The exact value of grid voltage is not important but the focus adjustment is critical.
9. Apply power to the Helmholtz coils.
BE SURE YOU START WITH MINIMUM CURRENT IN THE COILS WITH THE KNOB FULLY COUNTERCLOCKWISE.
Adjust the current in the coils until the electron beam bends in a complete semicircle and strikes the outer circle of the disk in the tube. Record the orbit radius and the coil current for which this condition obtains in cells D24 and E24, respectively.
10. Now increase the current in the Helmholtz coils until the electron beam strikes the next inner circle on the disk. Record the orbit radius and the coil current for this situation in cells D25 and E25.
11. Increase the coil current to get the electron beam to the third circle from the outside edge of the disk. Again record the orbit radius and the coil current in cells D26 and E26.
DO NOT ATTEMPT TO GET THE ELECTRON BEAM TO THE INNER CIRCLE. THE MAXIMUM CURRENT IN THE COILS AS READ ON THE AMMETER SHOULD NOT EXCEED 4.7 AMPS.
12. You have completed the data taking part of the experiment and are now ready to "turn off" the equipment.
Reduce the coil current to zero.
Next reduce the plate and grid voltages to zero.
Finally, turn off the filament power.
13. Print out the completed Worksheet and use it to
develop your lab report.