Page:Elementary Text-book of Physics (Anthony, 1897).djvu/133

§ 106] the change in the pressure. The modulus differs for different liquids.

The instrument used to determine the modulus of elasticity of liquids is called a piezometer. The first form in which the instrument was devised by Oersted, while not the best for accurate determinations, may yet serve as a type.

The liquid to be compressed is contained in a thin glass flask, the neck of which is a tube with a capillary bore. The flask is immersed in water contained in a strong glass vessel fitted with a water-tight metal cap, through which moves a piston. By the piston, pressure may be applied to the water, and through it to the flask and to the liquid contained in it.

The end of the neck of the small flask is inserted downwards under the surface of a quantity of mercury which lies at the bottom of the stout vessel. The pressure is registered by means of a compressed-air manometer (§ 124) also inserted in the vessel. When the apparatus is arranged, and the piston depressed, a rise of the mercury in the neck of the flask occurs, which indicates that the water has been compressed.

An error may arise in the use of this form of apparatus from the change in the capacity of the flask, due to the pressure. Oersted assumed, since the pressure on the interior and exterior walls was the same, that no change would occur. Poisson, however, showed that such a change would occur, and gave a formula by which it might be calculated. By introducing the proper corrections. Oersted's piezometer may be used with success.

A different form of the instrument, employed by Regnault, is, however, to be preferred. In it, by an arrangement of stopcocks, it is possible to apply the pressure upon either the interior or exterior wall of the flask separately, or upon both together, and in this way to experimentally determine the correction to be applied for the change in the capacity of the flask.

It is to be noted that the modulus of elasticity for liquids is so great, that, within the ordinary range of pressures, they may be