Final answer:
Thermometers rely on reaching thermal equilibrium with the system they measure, based on the zeroth law of thermodynamics, to provide accurate temperature readings.
Step-by-step explanation:
The assertion that thermometers are useless because they only register their own temperature overlooks the fundamental principles of thermal equilibrium and the zeroth law of thermodynamics. When a thermometer is placed in contact with another system (like a person's body), heat is transferred until both the thermometer and the system are at the same temperature, reaching thermal equilibrium. At this point, no further heat transfer takes place, and the temperature displayed by the thermometer accurately reflects the temperature of the system it has been in contact with.
Digital medical thermometers, for instance, use a component called a thermistor, which changes resistance with temperature. By measuring this resistance at thermal equilibrium, the thermometer can provide an accurate reading of the patient's body temperature. This demonstrates how thermometers are designed on a consequence of the zeroth law of thermodynamics—namely that if two systems are each in thermal equilibrium with a third system, all three systems are in thermal equilibrium with each other.