Instruction Manual and
Experiment Guide for the
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PASCO scientific
Model AP-8215
GRAVITATIONAL TORSION BALANCE
AP-8215
GRAVITATIONAL
TORSION BALANCE
Attach to
Earth Ground.
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Gravitational Torsion Balance 012–06802B
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F
m1m
r
head of
torsion ribbon
Introduction
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.
grounding
wire
large
masses
base with leveling feet
zero adjust
knob
mirror on
pendulum bob
sight for
leveling
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
Newton’s law of universal
gravitation:
2
2
where m
= G
and m2 are the masses of
1
the objects, r is the distance between
them, and
G = 6.67 x 10
-11
Nm2/kg
2
1
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Equipment
Included:
•Gravitational Torsion Balance
•support base with leveling feet
•1.5 kg lead balls (2)
•plastic plate
•replacement torsion ribbon
(part no. 004-06788)
•2-56 x 1/8 Phillips head screws (4)
•Phillips screwdriver (not shown)
Additional Required:
•laser light source (such as the PASCO OS-9171 He-Ne Laser)
•meter stick
2-56x1/8
Phillips head
screws
torsion ribbon
head
replacement
torsion ribbon
Attach to
Earth Ground.
AP-8215
GRAVITATIONAL
TORSION BALANCE
zero adjust
knob
aluminum
plate
pendulum
mirror
1.5 kg lead
masses
plastic
demonstration
plate
Figure 2
Equipment Included
2
optical grade
glass window
large mass
swivel support
leveling sight
leveling feet
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Attach to
Earth Ground.
GRAVITATIONAL
TORSION BALA NCE
AP-8215
IMPORTANT NOTES
Equipment Parameters
•Small lead balls
Mass: 38.3 g
Radius: 9.53 mm
Distance from ball center to torsion axis: d = 50 .0 mm
•Large lead balls
Mass: 1500 g
Radius: 31.9 mm
•Distance from the center of mass of the large ball to the
center of mass of the small ball when the large ball is
against the aluminum plate and the small ball is in the
center position within the case: b = 46.5 mm (Tolerances
will vary depending on the accuracy of the horizontal
alignment of the pendulum.)
•Distance from the surface of the mirror to the outer surface
of the glass window: 11.4 mm
•Torsion Ribbon Material: Beryllium Copper
Length: approx. 260 mm
Cross-section: .017 x .150 mm
+ 0.2 g (m2)
+ 10 g (m1)
➤ The Gravitational Torsion Balance
is a delicate instrument. We
recommend that you set it up in a
relatively secure area where it is safe
from accidents and from those who
don’t fully appreciate delicate
instruments.
➤ The first time you set up the
torsion balance, do so in a place
where you can leave it for at least one
day before attempting measurements,
allowing time for the slight elongation
of the torsion band that will occur
initially.
Keep the pendulum bob secured in
the locking mechanisms at all times,
except while setting up and
conducting experiments.
Equipment Setup
Initial Setup
1. Place the support base on a flat, stable table that is located
such that the Gravitational Torsion Balance will be at least
5 meters away from a wall or screen.
Note: For best results, use a very sturdy table, such as an optics
table.
2. Carefully remove the Gravitational Torsion Balance from
the box, and secure it in the base.
3. Remove the front plate by removing the thumbscrews
(Figure 3), and carefully remove the packing foam from the
pendulum chamber.
Note: Save the packing foam, and reinstall it each time the
Gravitational Torsion Balance is transported.
4. Fasten the clear plastic plate to the case with the
thumbscrews.
Do not touch the mirror on the pendulum.
pendulum
chamber
pendulum
bob
aluminum plate
Figure 3
Removing a plate from the chamber box
3
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torsion ribbon
Leveling the Gravitational Torsion Balance
head
1. Release the pendulum from the locking mechanism by
unscrewing the locking screws on the case, lowering the
locking mechanisms to their lowest positions (Figure 4).
torsion ribbon
!
Turn locking
screws clockwise.
"
pendulum
bob arm
locking
mechanisms
Figure 4
Lowering the locking mechanism to release the pendulum bob arms
2. Adjust the feet of the base until the pendulum is centered in
the leveling sight (Figure 5). (The base of the pendulum will
appear as a dark circle surrounded by a ring of light).
3. Orient the Gravitational Torsion Balance so the mirror on
the pendulum bob faces a screen or wall that is at least 5
meters away.
SIDE,
CUTAWAY
VIEW
pendulum
Pendulum bob
must be
Look through the
sight to view the
reflection of the
pendulum bob in
the mirror.
centered over
the mirror.
mirror
Figure 5
Using the leveling sight to level the
Gravitational Torsion Balance.
Vertical Adjustment of the Pendulum
The base of the pendulum should be flush with the floor of the
pendulum chamber. If it is not, adjust the height of the pendulum:
1. Grasp the torsion ribbon head and
loosen the Phillips retaining screw
Grasp the torsion ribbon head
and loosen the Phillips screw.
(Figure 6a).
2. Adjust the height of the pendulum
by moving the torsion ribbon head
up or down so the base of the
pendulum is flush with the floor of
the pendulum chamber
(Figure 6b).
3. Tighten the retaining (Phillips
head) screw.
Figure 6
Adjusting the height of the pendulum bob
4
The bottom of the pendulum
bob should be flush with the
!
floor of the chamber.
"
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