Pressure

Pressure, the action of a force applied to a specific surface. The action of a force on a solid body in a direction perpendicular to the surface produces normal pressure on the body's surface. The surface of a solid body is under the P. of the surrounding air. Atmospheric pressure also acts on the surface of a liquid. The surface of a liquid is horizontal, with the forces acting on each particle of liquid at the surface directed along the normal to the surface. If a liquid is placed in a cylinder and a piston is placed above it, then by applying force to this piston, pressure is produced on the liquid. The transmission of pressure in a liquid occurs uniformly in all directions, and the P. produced on 1 cm² of surface in one part of the liquid is transmitted equally to all other parts of the liquid. This is used to create large P. by applying a small force. For this purpose, liquid is placed in communicating vessels (see Figure 1), one limb of which (limb A) has a large surface area, while the other has a small surface area (limb B). By applying a certain force to the small surface B, a force that is many times greater is obtained on the large surface A, by as many times as one surface is larger than the other. By taking the surface of the larger piston P to be millions of times larger than the surface of the small piston Q, enormous P. can be obtained in P. Hydraulic presses, which are of great importance in technology, are based on this principle. Gases have exactly the same properties if they are enclosed in vessels into which a piston fits. By pushing the piston into the vessel containing the gas, enormous pressure can be obtained. By using communicating vessels, gas presses can be created, which are not used in technology because they are extremely dangerous in case of rupture of the cylinder in which the gas is contained. (For the measurement of P.-see Manometer.)-Osmotic pressure. If an aqueous solution of sugar is placed in vessel A (see Figure 2), consisting of a porous cylinder whose pores are impregnated with ferrocyanide copper, which does not allow salts to pass through but allows water to pass through, and if this vessel is placed in a large vessel with water B, then it can be observed that in tube C, which closes the porous cylinder, the liquid will rise. This proves that there is a constant influx of liquid into the cylinder, and the liquid will establish itself at a certain definite level, which will be the higher the greater the concentration of sugar C inside vessel A. If solutions of different concentrations are taken, then the P., measured by the height of the column h, will be equal to a certain constant value R multiplied by the concentration C. If the temperature t is changed simultaneously while keeping the concentration constant, then the P. will increase proportionally to the absolute temperature t + 273. Thus, if we denote by T the absolute temperature, by t the temperature on the Celsius scale, and by p the pressure of the column h of liquid, then for osmotic P. we obtain the final relationship: p=RCT. In this equation, concentration is expressed in gram-molecules per unit volume. The unit divided by C corresponds to the volume v occupied by one gram-molecule. Thus, the law can be written differently: pv=RT. The law written in this form is identical with the law of Mariotte and Gay-Lussac, and it is remarkable that R will correspond to the value of the gas constant in the law of Boyle-Mariotte and Gay-Lussac.

P. Lazarev. Abnormal P. (increased and decreased) atmospheric has sanitary-hygienic significance for a number of professions. Increased atmospheric P. is experienced by divers and caisson workers; in conditions of decreased P. it is necessary for aviators and aeronauts to remain for extended periods. Since in both cases increased requirements are made mainly on the cardiovascular system (as well as on respiratory and other organs), the main attention here must be paid to preliminary professional selection and constant medical observation. For these works, special detailed rules have been developed that precisely regulate both professional selection and work regimen. (See

Aviation Hygiene, Diving Work, Caisson Work, Decompression Illnesses.)