or, neglecting the term containing e~,we have A'~eM'.

(7)

The additional weight can be made in the form of a thin wire ring or a rider suspended together with the tare from the pan hook. The density of the tested liquid is d e t e r m i n e d in the following order. An equilibrium is first obtained in b a l ancing the float in air. The a b o v e - m e n t i o n e d additional weight is added to the tare and the float weighed in the tested liquid, i.e., the mass AM1 of weights which b a l a n c e the float in the liquid is determined at t e m p e r a t u r e t . From the knowledge of AM 1 and M' the density of the liquid can then be c a l c u l a t e d from (6). In cases of a large difference in surface tension between the tested liquid and water, if the calibration has been made in water, a correction for c a p i l l a r i t y is obtained and applied in the usual manner. The densities of several fairly viscous oils taken in small sample quantities (about 100 cm s) were measured by means of this method in the measurements of mass laboratory of the VNIIM (All-Union Scientific Research Institute of Metrology). The "Gosmetr" b a l a n c e type ADV-200 with a selected float was first c a l i b r a t e d by the above.mentioned method. The float of an approximate volume of 10 cm 3 had a platinum suspension of 0.05 mm in diameter, The b a l a n c e was c a l i b r a t e d in water at room t e m p e r a t u r e . The mean value of mass M' of the weights was equal to 9.9969 g. The m a x i m u m error in these measurements was about ~:0.0001~ g. This error will provide On determining with the given volume of the float the density of the liquid)an error of the order of 2 . 1 0 -~ g / c m 3, which is negligibly s m a l l as c o m p a r e d with the error in determining the density of viscous liquids, for instance, oil. The table gives results of measurements of density of the oil samples submitted. It will be seen from the table, that the deviations of the measured densities from their m e a n value in the m a jority of cases do not e x c e e d 5" 10 -s g / c m s. The m a x i m u m error in a number of measurements of the density of these samples did not exceed 1 9 10 -4 g / c m 3. Hence, the error in determining the density even of sufficiently viscous liquids was s m a l l e r than that obtained on special hydrostatic balances. This error can even be s m a l l e r in determining the density of nonviscous liquids, for instance, of aqueous solutions of alcohol, providing the temperature of the liquid does not vary during measurements. Thus, it is possible to r e c o m m e n d b a l a n c e ADV-200, which is mass-produced by our industry, for determining the density of s m a l l quantities of liquids.

1.

LITERATURE Transactions of the VNIIM, No. 37(97) (1959).

EFFECT

OF A N E L E C T R I C A L

OF L A B O R A T O R Y

FIELD

ON T H E

CITED

READINGS

BALANCES

A. i , Grebenshchikov

and

I.

T.

Kraev

Translated from I z m e r i t e l ' n a y a Tekhnika, No. 8, p. 30, August, 1962 The t e c h n i c a l control division of the "Fizpribor" plant has discovered in checking e q u a l - a r m 2nd class (physic o t e c h n i c a l ) balances with a measurement tolerance of 15 rag, that in an unloaded state they changed their r e a d ings at times by 850-1000 rag. Such variations were not observed, however, in the absence of the wooden stand on which the b a l a n c e is mounted. Subsequent tests showed that these variations were due to an e l e c t r o s t a t i c field produced by the stand being wiped with a dry hand or a dry rag.

657

The Physics Department of the Kirov Agricultural Institute, which helped to e l i m i n a t e the above defect, has come to thc conclusion that the electrification of the stand was due to the low conductance of the wood covering used by the plant (nitro-lacquer over a nitro-foundation), and suggested that its conductance be raised by the a d d i tion of highly-conducting substances. An attempt to raise the conductance by adding aluminum powder did not yield the expected results and, hence, it was decided to use for the purpose ordinary table salt in various quantities (1.8-5~/o). Experiments have shown that it is sufficient to add 1% salt to the nitro-laquer and nitro-foundation, since the variation in the balance readings did not then exceed the tolerance and the quality of the surface covering had not deteriorated. The surface b e c a m e e x cessively rough when 3 or 5% salt was added. At the same t i m e work was carried out to find a substitute for wood as a m a t e r i a l for the instrument stand. Brand K-17 and K-18 carbolite was found suitable for this purpose. Stands m a d e from this m a t e r i a l have a smooth surface and ca~mot be electrified. In this connection the plant has discontinued the more difficuIt production of wooden stands and is now using carbolite.

858