What the zero signifies becomes clear directly we start to work with thermometers not marked with scales. For in order to be used in this form as real thermometers, they must be exposed on each occasion first of all to some zero level of temperature, say, that of melting ice. If we then take them into the region of temperature we want to measure, we shall discern the difference of levels through the corresponding movement of the column. The final position of the column tells us nothing in itself. It is always the change from one level to another that the thermometer registers - precisely as does the sense of warmth in our hands in the experiment just described.