Experiment: Training Oscilloscope II (230 V, 50/60 Hz)

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Thematic focus:_..

•	Superposing magnetic fields with the same and different frequencies, and observing the displacement of the focused spot on the screen of the tube.
•	Checking the frequency of the mains supply.
•	Generating closed Lissajous’ figures.

Experiment description:_..

In the experiment, two identical current-carrying coils are placed outside the Braun tube, and the deflection of the electron beam in the magnetic fields of the coils is observed on the tube’s fluorescent screen as shifts of the focused spot. The magnetic field of the horizontal coil causes a vertical shift of the beam, while that of the vertical coil causes a horizontal shift.
When an alternating magnetic field at the mains frequency is applied to one of the coils, the focused spot is stretched out to become a vertical or horizontal line. If both coils are then connected in parallel to the alternating voltage source, the screen shows a straight line at 45° to the vertical, whereas when the coils are connected in opposition the line is at -45°, as the shifts produced by the two magnetic fields are superposed.
The experiment can be extended to study the effects of alternating magnetic fields of different frequencies. The Lissajous’ figures that then appear on the screen depend critically on the relationship between the frequencies of the two magnetic fields and on their phase relationship. When the ratio of the frequencies is an integer or a simple fraction, closed figures are generated. Their exact shape also depends on the phase difference between the magnetic fields.