weights appear in prehistoric graves in Egypt, dating from about 4000 B.C. The oldest methods
of weighing show stone weights being used as balances to weigh gold in Egyptian temple
treasures. These records date from about 2500 B.C. Referring to the years just mentioned, what
would the people use in that period for weight standards? They could have used almost anything
for a standard. The Babylonians used seeds of grain as weights. The Romans claimed 6,992
grains of wheat were equal to one pound (Libra). The English, using the same system, rounded
the amount of wheat for one pound to be equal to 7,000 grains of wheat. A unit of weight called
the gram was originally equal to the mass of one cubic centimeter of pure water at maximum
density (approximately 4o C) in a vacuum. The present definition of a gram is the mass
equivalent to one thousandth of the international prototype kilogram which is kept at the
International Bureau of Weights and Measures at Sevres, France.
b. As you noticed in the definition of a gram, the word mass rather than weight was used. Mass
is a measure of the quantity of matter contained in a body. Mass is a characteristic of a body
which will not change until the nature of the body is changed. Weight, on the other hand, is a
measure of both the mass of a body and the pull of gravity on the body. Weight will change
whenever the nature of the body is changed or the gravitational pull on the body is changed. The
pull of gravity on a body changes when the distance of the body from the center of the earth
changes. The pull of gravity on a body at the top of high mountain is less than the pull on the
same body if it were at sea level. Therefore, the weight of the body measured on the top of the
mountain will be less than its weight measured at sea level, even though its mass is exactly the
same in both locations. If weight were used as a standard, it would be of little value because it
would have different values in different locations. A mass standard, on the other hand, will have
the same value in any location whether it is used at Redstone Arsenal, Alabama or by an
astronaut on the moon. These standard masses are commonly called weights, but remember that
they are mass standards and not weight standards.
4. Mass and Its Measurement
a. The mass of an object is a measure of its inertia. Inertia is usually defined as that
general property of matter which causes a body in motion to remain in motion or a body
at rest to remain at rest unless acted upon by an external force. Any discussion on mass
and balances must also include a discussion of weights, since in our gravitational
environment any mass always exhibits the property of weight. The relationship between
mass and weight is so inseparable that mass determinations by means of comparing
unknown weights to reference standards are referred to as "weighing processes."
b. Mass Standards. The metric standard of mass is the kilogram. Smaller units are used
in the measurements you perform. Mass standards similar to the one in figure 1 are
generally called weights. The National Bureau of Standards, in order to identify weights
as to their general precision, stability, and use, has set up classifications for different
weights. Although not mandatory, this scheme has found wide acceptance in commerce
and industry. Weights, as described by the National Bureau of Standards (NBS), are
divided into four basic classifications, which are:
(1) Precision Laboratory Standards.
(2) General Laboratory Weights.
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