Study of a cistern/mercury barometer and to measure the atmospheric pressure.

Apparatus:-

Cistern barometer.

Construction of Cistern Barometer:-

It is a mercury barometer in which the lower mercury surface is larger in area than the upper surface. The basic construction of a cistern barometer is as follows:

A glass tube 1 m in length, sealed at one end, is filled with mercury, and then inverted. The tube is mounted so that its mouth penetrates the upper surface of a reservoir of mercury called the cistern of the barometer. Cistern barometers are classified according to whether the cistern is fixed in volume (Kew barometer) or variable in volume (Fortin barometer).

Working of Cistern Barometer:-

The barometer shall be mounted in a vertical position in accordance with applicable instructions. The observer shall adjust the cistern as follows:

(1). Tap the barrel near the top of the mercury column.

(2). Turn the thumbscrew at the bottom of the barometer until the surface of the mercury in the cistern touches the tip of the ivory point (i.e., until the top coincides with its image in the mercury). If a dimple forms on the surface, indicating that the mercury has been raised too far, turn the thumbscrew in the opposite direction until the dimple disappears and the ivory point coincides with its image in the mercury. Contact of the mercury with the ivory point is more easily seen against a white background.

(3). Set the vernier so that the base just cuts off light at the highest point of the meniscus (the curved upper surface of the mercury column. A white background facilitates this setting.

(4). Lower the mercury about 1/4 inch from the ivory point; do not change the setting of the vernier.

Mercury Barometer:-

A mercury barometer has a glass tube with a height of at least 84 cm, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.