I'm confused by how thermometer works based on The Zeroth Law of Thermodynamics. The Zeroth Law of Thermodynamics said that

"If a body, A, be in thermal equilibrium with two other bodies, B and C, then B and C are in thermal equilibrium with one another."

Then, about thermal equilibrium, it said that

"If when two bodies are placed in thermal communication, one of the two bodies loses heat, and the other gains heat, that body which gives out heat is said to have a higher temperature than that which receives heat from it."

Then if we use a thermometer to measure our body temperature, if the temperature of our body is higher than the temperature of the liquid inside the thermometer, will the heat flow from our body to the liquid inside the thermometer to reach the thermal equilibrium? If yes, then will our body lose heat and temperature of our body decrease?

How does the Zeroth Law of Thermodynamics on Thermometer actually works?

  • 1
    $\begingroup$ A good thermometer is designed to have a very small heat capacity, so it can probe the temperature of a system without the need to exchange too much heat. In this way, the system's temperature stays constant. $\endgroup$
    – Quillo
    Nov 29, 2023 at 9:35

1 Answer 1


You place a thermometer in thermal contact with system $A$ and get a "reading" $R$ on the thermometer.
As an example $R$ could be a mark at the position of the top of the mercury column in a mercury in glass thermometer noting that in this case no number is involved.
The thermometer is now placed in thermal contact with system $B$.
If the level of the mercury column is the same as it was when in thermal contact with system $B$ we can say that the temperatures of systems $A$ and $B$ is the same, and if systems $A$ and $B$ were placed in thermal contact with one another there would be no net heat transfer between them, ie they would be in thermal equilibrium. Thus, the same reading on a thermometer means that bodies have the same temperature.

From that idea you can then devise a temperature scale to measure the temperatures of bodies and ascertain which body had the higher temperature, ie which way "heat" would flow if the two bodies were in thermal contact.

For example you could have an uncalibrated mercury in glass thermometer and put a series of marks on the capillary and label them $a,\,b,\,c\,,$ etc with $a$ the mark nearest the bulb.
On day $1$, you place the thermometer under your tongue and the mercury is at level $b$.
On day $2$ the mercury level is at level $c$ which would mean that your temperature has increased from that on day $1$.
On day $3$ the mercury level is at level $a$ which would mean that your temperature has decreased to its lowest value in the three days.
This is not a very practical temperature scale does enable you to compare your body temperature (system $A$) and the temperature of others (system $B$).


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