PASCO AP-8215, Инструкция по эксплуатации, Страница: 5 / 24

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1
I n t r odu c t i on
The PASCO scientific AP-8215 Gravitational Torsion Balance
reprises one of the great experiments in the history of physics—
the measurement of the gravitational constant, as performed by
Henry Cavendish in 1798.
The Gravitational Torsion Balance consists of two 38.3 gram
masses suspended from a highly sensitive torsion ribbon and two
1.5 kilogram masses that can be positioned as required. The
Gravitational Torsion Balance is oriented so the force of gravity
between the small balls and the earth is negated (the pendulum is
nearly perfectly aligned vertically and horizontally). The large
masses are brought near the smaller masses, and the gravitational
force between the large and small masses is measured by
observing the twist of the torsion ribbon.
An optical lever, produced by a laser light source and a mirror
affixed to the torsion pendulum, is used to accurately measure the
small twist of the ribbon. Three methods of measurement are
possible: the final deflection method, the equilibrium method, and
the acceleration method.
A Little Background
The gravitational attraction of all objects toward the Earth is
obvious. The gravitational attraction of every object to every
other object, however, is anything but obvious. Despite the lack
of direct evidence for any such attraction between everyday
objects, Isaac Newton was able to deduce his law of universal
gravitation.
However, in Newton's time, every measurable example of this
gravitational force included the Earth as one of the masses. It was
therefore impossible to measure the constant,
G , without first
knowing the mass of the Earth (or vice versa).
The answer to this problem came from Henry Cavendish in 1798,
when he performed experiments with a torsion balance,
measuring the gravitational attraction between relatively small
objects in the laboratory. The value he determined for
G allowed
the mass and density of the Earth to be determined. Cavendish's
experiment was so well constructed that it was a hundred years
before more accurate measurements were made.
Figure 1
Assembled Gravitational Torsion Balance,
ready to begin Henry Cavendish’s classic
experiment to determine the gravitational
constant
base with leveling feet
grounding
wire
large
masses
mirror on
pendulum bob
head of
torsion ribbon
zero adjust
knob
Newton’s law of universal
gravitation:
where m
1
and m
2
are the masses of
the objects, r is the distance between
them, and
G = 6.67 x 10
-11
Nm
2
/kg
2
F = G
m
1
m
2
r
2
sight for
leveling