A thermometer is an excellent application of the laws of physics. A glass bulb filled with mercury or alcohol. The volume of the liquid depends on the temperature. The warmer the liquid, the higher it rises in the bulb. Simple and reliable.
To draw a thermometer, it's enough to depict a bulb. At the bottom, there's an expansion for the liquid, above it - a narrow elongated tube along which a scale is marked. You can add a casing around it, or do without it.