PASCO AP-8215A Gravitational Torsion Balance - Manual (Page 5)

®
M o d e l N o . A P - 8 2 1 5 A I n t r o d u c t i o n
1
Introduction
The PASCO scientific AP-8215A Gravitational Torsion Balance
reprises one of the great experiments in the history of physics—the
measurement of the gravitational constant, as performed by Henry Cav-
endish 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 gravita-
tional 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 deter-
mined. Cavendish's experiment was so well constructed that it was a hun-
dred years before more accurate measurements were made.
Base with leveling feet
Large
masses
Sight for
leveling
Grounding
wire
Head of
torsion ribbon
Zero adjust
knob
Mirror on
pendulum
bob
Figure 1: Assembled Gravitational
Torsion Balance, ready to begin Hen
Cavendish’s classic experiment to
determine the gravitational constant
Newton’s Law of Universal Gravitation:
where m
1
and m
2
are the masses of the
objects, r is the distance between their
centers, and G is the universal gravita-
tional constant, 6.67 x 10
-11 Nm 2 /kg 2 .
F G
m
1
m
2
r
2
-------------- =