Page:Elementary Text-book of Physics (Anthony, 1897).djvu/144

130 The form of barometer first made by Torricelli is still often used, especially when the instrument is stationary, and is intended to be one of precision. In the finest instruments of this class a tube is used which is three or four centimetres in diameter, so as to avoid the correction for capillarity. A screw of known length, pointed at both ends, is arranged so as to move vertically above the surface of the mercury in the cistern. When an observation is to be made, the screw is moved until its lower point just touches the surface. The distance between its upper point and the top of the column is measured by means of a cathetometer; and this distance, added to the length of the screw, gives the height of the column.

Other forms of the instrument are used, most of which are arranged with reference to convenient transportability. Various contrivances are added by means of which the column is made to move an index, and thus record the pressure on a graduated scale. All these forms are only modifications of Torricelli's original instrument.

The pressure indicated by the barometer is usually stated in terms of the height of the column. Mercury being practically incompressible, this height is manifestly proportional to the pressure at any point in the surface of the mercury in the cistern. The pressure on any given area in that surface can be calculated if we know the value of $$g$$ at the place and the specific gravity of mercury, as well as the height of the column. The standard barometric pressure, represented by 760 millimetres of mercury, is a pressure of 1.033 kilograms on every square centimetre. It is called a pressure of one atmosphere; and pressures are often measured by atmospheres.

In the preparation of an accurate barometer it is necessary that all air be removed from the mercury; otherwise it will collect in the upper part of the tube, by its pressure lower the top of the column, and make the barometer read too low. The air is removed by partially filling the tube with mercury, which is then boiled in the tube, gradually adding small quantities of mercury, and boiling