![]() The relative humidity was measured inside the balance case with an electrical resistance type hygrometer. Later a thermistor thermometer was constructed which could be inserted inside the balance case. Obviously, the error that the uncertainties in the above quantities introduce in the weight determination, is proportional to the difference in volume of the object and the weights.ĭuring this experiment, the air temperature was measured to ☐.1 ☌ with a thermometer inserted inside the aluminum shroud of the balance. Under normal atmospheric conditions each of the following variations will produce approximately one μg/cm 3 change in the density of air: 0.6-mm change in barometric pressure, 0.2 ☌ change in temperature, and 7-percent change in relative humidity at 25 ☌. The following paragraphs describe the equipment and procedures used in this experiment. This fact was utilized in this experiment and has also been applied by various other observers who have coated their suspensions with either a rough gold or platinum black coating. On the other hand, the contact angle against a rough metal surface may be zero due to capillary attraction of the liquid to the surface. The associated problems of this method are that a zero contact angle is very difficult to attain, it depends on the direction of travel of the contact line relative to the metal surface, and minute traces of foreign matter on the metal surface will alter the contact angle considerably. It has also been suggested that the contact angle should be observed during the measurements to insure its constancy. Therefore the usual procedure in hydrostatic weighing has been to clean the suspension wire very carefully and to use exactly the same portion of the wire in each weighing. Usually, for a receding water surface, the contact angle is very nearly equal to zero. In general, the contact angle varies with the cleanliness of the metal surface and with the direction of motion of the water surface. Since this force on the wire varies as the cosine of the liquid contact angle, it is desirable to maintain a nearly zero contact angle, thus minimizing the force variations due to small changes in the angle. wire is wetted perfectly, the downward force on it due to surface tension is 0.574 dyn, which is equivalent to 586 μg of weight. The surface tension of water at 25 ☌ against air is 71.97 dyn/cm. Īs already stated, the main difficulty with the weighings in water is due to the surface tension effects on the suspension at the liquid surface. A good discussion of the weighing process and the construction of precision balances has been presented by Corwin. ![]() This accuracy depends on the precision and sensitivity of the balance, the accuracy of the weights, and the care with which the weighings are made. Thus the accuracy of the density determination depends to a large extent on the accuracy of the weighings. Where M 0 w is its apparent mass in water, and d H 2 O the density of water at the time of weighing. In another paper the result of this density determination has been used in conjunction with x-ray lattice spacing measurements of the same crystals to determine the absolute scale of x-ray wavelengths. Primary attention is paid to the apparatus and methods used for the hydrostatic weighing in water. This paper describes the procedure which has been used to measure the density of silicon crystals up to 18 g in size. These effects must be minimized by using minimum diameter suspension wire and by keeping a constant contact angle of the liquid against the wire. Aside from the usual problems associated with precision weighing, the main difficulty encountered is due to the variation of the surface tension effects of the liquid on the suspension wire passing through the surface. One of the most direct methods of determining the density of solid bodies is by hydrostatic weighing in a liquid of known density, and conversely, the density of a liquid can be measured by hydrostatic weighing of a solid body of known mass and volume. In a different type of experiment, the density of a crystal has been used as a factor in determining e/m of the electron from the index of refraction of x rays. Density has also been used as an indication of the lattice perfection of crystals. 4 Similarly, it is possible to determine relative atomic or molecular weights from the densities and lattice constants of different crystals. For instance, the density together with the lattice spacing and molecular weight of a crystal can be used to determine Avogadro’s number. ![]() Measurement of density is one of the most basic of physical measurements, and is often intrinsic in the determination of other physical constants.
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