Weighing

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Weighing

 

the determination of the mass of bodies by means of balances. A high degree of accuracy in weighing is achieved by taking into consideration all possible errors of the balances, the weights, and the weighing method used, as well as errors caused by external conditions, such as the influence of aerostatic, electric, and magnetic forces and variations in air temperature and humidity. (The limits of permissible error for balances of various types and for weights are given in the articles BALANCE and WEIGHTS.)

When a high degree of accuracy is not required and when the effects of aerostatic and other forces are not considered, the direct weighing method is usually used: the mass of a body is taken as equal to the algebraic sum of the mass of the weights that balance the body and of the readings on the balance. In this case, weighing on an equal-arm balance includes errors because of the inequality of the balance beam. Greater accuracy in direct weighing is achieved on a one-arm balance, which eliminates this error, since the body to be weighed and the balancing weights are on the same side of the balance beam. To completely eliminate error caused by inequalities of the balance beam when weighing on equal-arm balances, so-called accurate weighing methods are used.

The displacement method (Borda method) consists of the following steps: after balancing the body with a counter-weight (metal scraps, shot, and so on) placed on the opposite arm of the balance beam, the body is removed from the balance and replaced by weights such that the balance achieves the original equilibrium position. The mass of the body is determined by the mass of the weights needed to balance the scales and according to the reading on the balance corresponding to the part of the mass not balanced by the weights.

In the method of D. I. Mendeleev, one of the balance cups is filled with weights that correspond to the weight limit of the balance; a counterweight to balance the first weights is placed in the other cup. The body to be weighed is placed in the cup with the first weights, which are removed to the point at which the balance returns to its initial position of equilibrium. The mass of the body is determined by the amount of mass removed and by the reading on the scale.

The double weighing method (Gauss’ method) consists of a repeated direct weighing after transposing the body and the weights from one cup of the balance to the other. The mass of the body M = 1/2(M1+M2), where M1 and M2 are the results of two direct weighings. All three methods are equally accu-rate. The choice of a method depends on the construction of the balance and the weighing conditions. On balances of any type, weighing can be accomplished only with limited accuracy, since balances and weights always have a certain error.

Figure 1. Accuracy characteristics of mass measurements depending on size and the method of measurement: (1) comparison of the State Standard of Mass (the kilogram), (2) metrological research, (3) high-accuracy analytic research, (4) high-accuracy industrial weighing, weighings of precious metals, (5) commercial and economic measurements. The shaded area represents measurements made on automatic screen scales and other industrial scales and batchers.

Thus, on a balance with 0.1 percent error, it is impossible to weigh a body with less error. For particularly accurate weighing, accurate weighing methods are used, as well as calculating the error of the weights. To simplify evaluation of the error caused by aerostatic forces arising from the inequality of volumes of the measured body and the weights (Archimedes’ law), the specific density of the material is taken to be 8.0 x 103 kg/m3 (regardless of the material from which they are made) for all weights except standard weights. The accuracy of weighing in various branches of science, technology, and the national economy is shown in the form of a graph in Figure 1.

REFERENCES

Rudo, N. M. Laboratornye vesy i tochnoe vzveshivanie. Moscow, 1963.
Smirnova, N. A. Edinitsy izmerenii massy i vesa v Mezhdunarodnoi sisteme edinits. Moscow, 1966.

N. A. SMIRNOVA

References in classic literature ?
I don't weigh a bit more," said I, "nor as much; and my appetite may be better in the evening when you are here, but it is worse in the morning when you are away
Now, in getting under weigh, the station generally occupied by the pilot is the forward part of the ship.
Pa's got a few buckshot in him; but he don't mind it 'cuz he don't weigh much, any- way.
Huck was in a close place -- the inquiring eye was upon him -- he would have given anything for material for a plausible answer -- nothing suggested itself -- the inquiring eye was boring deeper and deeper -- a sense- less reply offered -- there was no time to weigh it, so at a venture he uttered it -- feebly:
You've got something of your father's happy disposition, or it would weigh on you as it does on me.
Teachers, you must watch her: keep your eyes on her movements, weigh well her words, scrutinise her actions, punish her body to save her soul: if, indeed, such salvation be possible, for (my tongue falters while I tell it) this girl, this child, the native of a Christian land, worse than many a little heathen who says its prayers to Brahma and kneels before Juggernaut--this girl is--a liar
Whether it was a son who would never know his father's story; who might even live to weigh the possibility of his father's having disappeared of his own will and act.
But if you were free to-day, to-morrow, yesterday, can even I believe that you would choose a dowerless girl -- you who, in your very confidence with her, weigh everything by Gain: or, choosing her, if for a moment you were false enough to your one guiding principle to do so, do I not know that your repentance and regret would surely follow.
Thomas Traddles, without acquitting myself of the pecuniary part of this obligation, would weigh upon my mind to an insupportable extent.
He keeps them on shelves over his head, and will weigh them all.
Think, then, and weigh thy words before thou dost answer, Slaughterer.
In these very scales shalt thou weigh me out,'' said the relentless Baron, ``a thousand silver pounds, after the just measure and weight of the Tower of London.