Electricity and Magnetism

Using a CRO to measure short time intervals

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

The separation along the x-axis between two pulses on a CRO (cathode ray oscilloscope) screen is a measure of the time interval between the pulses.

Apparatus and Materials

For each student group

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance


  1. Set the time base on the CRO to 1 ms/cm (1 ms/div) and the fine control at the slowest sweep speed. The gain (sensitivity) should be turned up to maximum.
  2. Attach the microphone to the input terminals of the oscilloscope. Give two short whistles (or claps) close to the microphone to see successive pulses a short interval apart. Vary the time interval between your two whistles and note the effect.
  3. Set the time base on the CRO to 10 ms/cm. Direct the microphone towards a wall, a few metres distant. Clap next to the microphone. The initial pulse and the reflected pulse will show up on the trace. Estimate the time between the pulses (you will need to know the setting of the time-base}, and measure the distance the signal travelled to the wall and back. You can now calculate the speed of sound.

Teaching Notes

  • Note that the CRO time-base will need to be set to its calibrated position if measurements of time intervals are to be made.

This experiment was safety-tested in October 2006

  • A video showing how to use an oscilloscope:

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