• You may use either the large or small e/m apparatus. The small apparatus has the advantage of not having to be aligned with the earth's magnetic field.

• Be sure to have your wiring checked before turning on the e/m apparatus. It is extremely important to exercise care when turning on the filament voltage. If the current gets too high through the filament it will burn out. You will note that as the filament voltage is applied, the current initially rises, then decreases. This is because the resistance of the filament increases as it warms up. Because of this non-ohmic behavior you should never turn off the filament power supply without turning down the voltage to the filament. If you turn the filament power supply back on with the voltage turned up there will be an initial surge of current through the filament that may well destroy it.

• Slowly dial up the filament current to about 0.7 amperes. After allowing the cathode to heat for a couple of minutes, reduce the filament current to 0.5 to 0.6 amperes

• Apply the plate potential  to accelerate the electrons, and with the lights off notice the blue stream of electrons that rise from the hole in the center of the disk. Adjust the plate potential to 60 - 100 volts and vary the grid potential to produce a narrow, sharply focused beam.

• Energize the Helmholtz coils and vary the current until the beam bends into a complete semicircle. Do not increase the current in the coils to the point where sparking occurs. Simultaneously adjust the plate potential to vary the accelerating potential difference and the field current until the beam falls on one of the marked circles (the Cenco e/m electron beam tube has marked circles of radii of 0.50, 1.0, 1.5 and 2.0 cm). The plate potential and field current must be adjusted together to keep the beam in sharp focus.

• Measure the mean radius of the Helmholtz coils and record the number of turns per coil (these may be counted). Acquire voltage and current data for each circle. If you are recording data for the circle of radius 1.0 cm, what is the radius of the electron beam?