KCSE 2025 Physics P1 Q6 — Clinical Thermometer Constriction (why the reading holds)
Published
The Question
“State one feature of a clinical thermometer that keeps the reading constant after it is removed from the patient's body.”
Locate the special feature in the tube
Just above the bulb the bore of a clinical thermometer is pinched into a very narrow section called the constriction, or kink. It is the one structural feature that separates a clinical thermometer from an ordinary laboratory one.
See what the constriction does on removal
While the thermometer is under the tongue the mercury expands and forces its way through the narrow constriction to record the body temperature. When the thermometer is taken away and the mercury begins to cool and contract, the thin thread of mercury snaps at the constriction. The mercury already above the kink cannot flow back into the bulb, so the reading stays exactly where it was until you shake it down.
Final Result
The feature is the constriction (kink) in the bore just above the bulb. It breaks the mercury thread when the thermometer is removed, holding the reading until the thermometer is shaken down.
Why this method works
A clinical thermometer must be read after it is taken out of the patient's mouth, so the mercury has to be prevented from retreating into the bulb the moment it starts to cool. The constriction is a deliberately narrow neck: mercury under the extra pressure of thermal expansion can be pushed forward through it, but the much weaker pull of contraction is not enough to draw it back. This one-way behaviour turns the thermometer into a maximum thermometer that latches onto the highest temperature it reached. Shaking the thermometer supplies the force needed to drive the mercury back through the constriction and reset it.
If there were no constriction the mercury would simply fall back into the bulb as it cooled, and the reading would drop before you could take it — exactly why laboratory thermometers, which are read in place, do not have one.