Strike the fork, hold it over the tube, and lower the water while you listen.
01Strike the fork on something soft and hold it flat over the open end of the tube.Struck on the bench it rings with overtones and chips; held edge-on the sound misses the tube. Flat over the mouth, a centimetre clear, is what drives the air column.
02Start with a short air column and lower the water steadily.Lengthening the column takes you up through the first resonance and on towards the second, in that order, so you always know which one you have just met.
03Listen for a swell rather than a click.The response builds over a centimetre or two. Expecting a sudden onset makes you move too fast and miss it.
04When it swells, keep going until it has clearly died away again.This is the step candidates skip. Without hearing it fall on the far side you can’t tell a maximum from a place that merely happened to be loud, and you’ll stop short of the peak.
05Come back slowly and settle on the loudest point between the two quiet places.Approaching from both sides brackets the peak, exactly as two opposite deflections bracket a null. The middle of what is left is your reading.
06Read the water level off the rule and record the air-column length.The rule gives you a millimetre; your ear gives you about a centimetre. Write down what the rule says, and know that the uncertainty belongs to the listening.
07Repeat for the second resonance, further down the tube.Two lengths let you subtract away the end correction. One length leaves it in your answer, unknown and about five per cent of it.
Laboratory
Strike the fork and hold it over the open end. The tube can’t resonate with nothing driving it.