The construction of Thermometer
Firstly, a dry and clean capillary glass tube of fine and uniform bore is taken. One end of it is connected to a cylindrical bulb B and the other end is closed. Now the tube is kept vertically straight by a stand and a funnel F is connected by a rubber tube to the open end of the capillary tube. Taking some pure and clean mercury in the funnel
the capillary tube is filled with mercury by successive heating the cooling the bulb. The volume of air inside the bulb and the tube expands due to heat and some air comes out through the mercury as bubbles. During cooling the volume of the rest air inside the bulb and the tube decreases, so some mercury enter inside the tube due to outside pressure. In this way, the bulb and the tube become filled with mercury. Then the bulb is heated so that mercury inside the bulb and the tube starts boiling and ejected vapour removes air from the tube. In this condition, the tube is sealed by a sharp and pointed flame. When the bulb is cooled to room temperature, the bulb and a part of the capillary tube become filled with mercury and the rest of the capillary tube remains empty. When the mercury returns to normal state, two marks are made on the tube corresponding to the temperatures of the two fixed points. In mercury thermometer, the lower fixed point is marked corresponding to the melting point of ice and the upper fixed point is marked corresponding to the boiling point of water. The difference of temperature between these two fixed points is called the fundamental interval. Depending on the temperature scale, the fundamental interval is equally divided into some divisions and each division is called degree.