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Effects of Temperature and Pressure and Gravity.Because the dimensions of any standard change with temperature, it is … (necessary, necessarily) to state the temperature at which standards of length, area, volume, or capacity are to be used, or, if used at any other than standard temperature, then the coefficient of expansion must be stated. The standard temperatures most often used are: 0 °C. (32 °F.), the standard temperature for the prototypes of the metric system, also used for some secondary metric standards and for some measurements; 20 °C. (68 °F.), the internationally adopted standard temperature which is being … (increasing, increasingly) used for weights and measures work; 16.67 °C. (62 °F.), the official temperature used in connection with the British imperial system; 4 °C. (39.20 °F.), the temperature of maximum density of water, used in density and volumetric work in which water is … (direct, directly) or … (indirect, indirectly) involved; 25 °C. (41 °F.), a standard temperature … (used, using) in some work in physical chemistry; and 15.56 °С. (60 °F.), the standard temperature … (used, using) in the petroleum industry for density and volume work.

Changes in … (atmospheric, atmospherically) pressure have very little effect on length standards, a change in pressure from 710 to 790 mm. causing a change in length of about 0.00005 mm. in the case of the prototype metre. Changes in … (atmospheric, atmospherically) pressure are usually disregarded in measurements of length.

Any change in air pressure, as well as changes in the temperature and humidity of the air is, however, of importance in any comparison of masses because these are factors … (affected, affecting) the buoyant effect of the air.

Any definitions of weights which involve comparisons of the weights of bodies having different densities are incomplete unless the atmospheric conditions are specified together with the densities of mate­rial employed. In the United States most commercial weights are verified on the basis of apparent mass in air against brass standards of density 8.4 g./cm.³ at 0 °C., no correction being made for the buoyant effect of the air, the values for the brass standards being their true mass or weight in vacuo.

When weighing on an equal arm balance, if the body being weighed does not have the same density as the weights, a correction must be made.

The standard air pressure usually … (used, using) in weights and measures work is 760 mm. mercury, at 20 °C., mercury having a specified density of 13.5951 g. per cm.³, with gravity 980.665 cm./sec.²

Since a spring balance indicates weight and not mass, a constant mass suspended from a spring balance will produce different readings when measurements are made in a series of places having sufficient changes in the force of gravity, the indicated weights varying … (direct, directly) with the force of gravity, other conditions being equal. An equal arm balance likewise really compares weights rather than masses, but on the assumption that the two pans of the balance are acted upon by the same force of gravity, the result is an … (indirect, indirectly) comparison of masses. If other conditions remained constant, a balance would every­where give the same balance between the same two bodies, no matter how the force of gravity varied from place to place — provided always as above, the assumption is justified that the force of gravity is the same on the two pans of the balance [2, С. 43 - 44].

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