Oscillation of a Galvanometer Coil - Forced Oscillations and Resonance


Ballistic galvanometer, Cathode Ray Oscilloscope, 10k resistance box, stop watch, mounted 20k resistor, signal generator, wires.


The natural frequency of oscillation of the galvanometer's coil is less than 1 Hz. These oscillations can be forced using the Unilab signal generator in the range 0 - 1 Hz. The galvanometer is using on its DIRECT ‘ballistic’ setting in order to reduce the effect of damping because of this you should take great care not to overload the instrument (repair costs approx. £200).

1. Connect up the circuit shown below BUT DO NOT PLUG THE SIGNAL GENERATOR INTO THE MAINS. For safety the circuit must be adjusted for you. Once it has been adjusted do not alter the output of the signal generator.

2. Set the frequency of the signal generator to 0.2Hz.

3. Determine the maximum amplitude Ao / mm of the oscillations of the coil. (The amplitude may vary over time.)

4. Use the stop watch and the oscilloscope's trace to measure the period of oscillation of the coil. Use this to calculate an accurate value of its frequency f / Hz. (This might be different from 0.2Hz as the scale on the signal generator is not very accurate.)

5. Repeat the above for signal generator frequency settings of 0.3Hz, 0.4Hz, 0.5Hz, 0.6Hz, 0.7Hz & 0.8Hz.

6. Tabulate your measurements and calculations.

7. Draw a graph of maximum amplitude,/mm against CALCULATED frequency,/ Hz.

8. From your graph find a value for the resonant frequency,of the coil.

9. What further measurements might you take to improve the accuracy of your value of the resonant frequency?

10. If you have time, note the effect of reducing the value of the resistance box to 1000 OHMS.

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