4.2.2 Preliminary Adjustments

The supplies and controls for the Helmholtz coils and the filament are permanently wired on a board and are designed to minimize the possibility of damage to the tube or coils. Locate each control and note the qualitative effects observed when the control is varied. In particular:

• Figure 1 shows that the filament and its associated lead wires form a small loop. Since a 4 amp current is required to heat the filament, this loop creates a measurable field. The filament coil reversing switch permits you to study the effect of this field. The effect can be minimized in the experiment by rotating the tube slightly in its mounting so that the electrons come out parallel to the plane of the coils. Will the electron velocities be changed and will the accuracy of the final result be jeopardized if this adjustment is not made carefully?
• Note the direction of the coil current for each position of its reversing switch by using the dip needle to check the direction of the resultant field. Knowing the field direction, check the sign of the particles in the deflected beam.
• The beam will have a slight curvature in the earth's field when the coil current is zero. Make a preliminary measurement of the earth's field by adjusting the coil current to remove this curvature. The special Meter Switch and low current meter will enable you to measure the relatively small current needed and the straight line trajectory can be checked by comparison with the light emitted from the filament.