Instruction Manual with Experiment Guide and Teachers’ Notes 012-11032B ® *012-11032* Gravitational Torsion Balance T G O R R A S V IO IT A N AT P B IO -82 A N 1 L A 5 A N L C E AP-8215A d.
Note: To avoid breaking the torsion ribbon, the locking mechanism must be fully raised on both sides during moving or transporting the Gravitational Torsion Balance. Note: Save the packing material from the interior of the box, and re-install this material during moving or transporting the Gravitational Torsion Balance.
Gravitational Torsion Balance Ta b l e o f C o n t e n ts Section Page Copyright, Warranty and Equipment Return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Equipment Parameters . . . . . . . . . . . . . . . . . . . .
Gravitational Torsion Balance C o p y r i g h t , W a r r a n t y , a n d E q u ip m e n t R e t u r n Copyright, Warranty, and Equipment Return Please—Feel free to duplicate this manual subject to the copyright restrictions below. Copyright Notice The PASCO scientific 012-11032A Gravitational Torsion Balance manual is copyrighted and all rights reserved.
Model No. AP-8215A Introduction 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 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.
Gravitational Torsion Balance Equipment Equipment Included: Gravitational Torsion Balance Plastic Plate Support Base with Leveling Feet Replacement Torsion Ribbon* 1.5 kg Tungsten Balls (2) 2-56 x 1/8 Phillips head screws (4) Adapter Rings (2) Phillips screwdriver (not shown) (*part no. 004-06788) Additional Required: Laser light source (such as the PASCO OS-9171 Helium-Neon Laser) Meter stick Torsion ribbon head Zero adjust knob 1.
Model No. AP-8215A Equipment Parameters • Small tungsten balls Mass: 38.3 g ±0.2 g (m2) Radius: 8.19 mm Distance from ball center to torsion axis: d = 50.0 mm • Large tungsten balls Mass: 1500 ±10 g (m1) Radius: 27.6 mm • Distance from the center 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.
Gravitational Torsion Balance Equipment Setup Leveling the Gravitational Torsion Balance 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 head A E tta a c rt h h to G ro u n d . 1. Torsion ribbon Side, Cutaway View Pendulum Figure 4: Lowering the locking mechanism to release the pendulum bob arms 2. 3.
Model No. AP-8215A Equipment Setup Rotational Alignment of the Pendulum Bob Arms (Zeroing) The pendulum bob arms must be centered rotationally in the case — that is, equidistant from each side of the case (Figure 7). To adjust them: 1. Mount a metric scale on the wall or other projection surface that is at least 5 meters away from the mirror of the pendulum. 2. Replace the plastic cover with the aluminum cover. 3.
Gravitational Torsion Balance Equipment Setup d.If the spots on the projection surface (the laser beam reflections) are not aligned vertically, loosen the zero adjust thumbscrew, turn the zero adjust knob slightly to refine the rotational alignment of the pendulum bob arms (Figure 10), and wait until the movement of the pendulum stops or nearly stops. e.Repeat steps 4a – 4c as necessary until the spots are aligned vertically on the projection surface. 6.
Model No. AP-8215A Measuring the Gravitational Constant Measuring the Gravitational Constant Overview of the Experiment The gravitational attraction between a 15 gram mass and a 1.5 kg mass when their centers are separated by a distance of approximately 42.2 mm (a situation similar to that of the Gravitational Torsion Balance ) is about 7 x 10 -10 newtons. If this doesn’t seem like a small quantity to measure, consider that the weight of the small mass is more than two hundred million times this amount.
METHOD I: Measurement by Final Deflection Since the system is in equilibrium, the twisted torsion band must be supplying an equal and opposite torque. This torque (band) is equal to the torsion constant for the band () times the angle through which it is twisted (), or: band = – 1.3 Combining equations 1.1, 1.2, and 1.
Model No. AP-8215A METHOD I: Measurement by Final Deflection Therefore: 2 2 2 d + --- r 5 = 8 m 2 -------------------2 T Substituting equations 1.5 and 1.8 into equation 1.4 gives: 2 2 2 2 d + --- r 2 2 5 G = Sb -------------------- 2 T m 1 Ld 1.8 1.9 All the variables on the right side of equation 1.9 are known or measurable: r = 9.55 mm d = 50 mm b = 42.2 mm m1 = 1.5 kg L = (Measure as in step 1 of the setup.
Gravitational Torsion Balance 5. METHOD I: Measurement by Final Deflection Use a stop watch to determine the time required for one period of oscillation (T). For greater accuracy, include several periods, and then find the average time required for one period of oscillation. Note: The accuracy of this period value (T) is very important, since the T is squared in the calculation of G. 6. Wait until the oscillations stop, and record the resting equilibrium point (S2). Analysis 1.
Model No. AP-8215A METHOD II: Measurement by Equilibrium Positions METHOD II: Measurement by Equilibrium Positions Observation Time: ~90+ minutes Accuray: ~5% Theory When the large masses are placed on the swivel support and moved to either Position I or Position II, the torsion balance oscillates for a time before coming to rest at a new equilibrium position.
Gravitational Torsion Balance METHOD III: Measurement by Acceleration METHOD III: Measurement by Acceleration Observation Time: ~ 5 minutes Accuracy: ~ 15% Theory With the large masses in Position I, the gravitational attraction, F, between each small mass (m2) and its neighboring large mass (m1) is given by the law of universal gravitation: m1 m2 F = G -------------2 b 3.1 This force is balanced by a torque from the twisted torsion ribbon, so that the system is in equilibrium.
Model No. AP-8215A M ET H O D I I I : M e a s u r e m e n t b y A c c e l e r a t i o n The acceleration is measured by observing the displacement of the light spot on the screen.
Gravitational Torsion Balance Maintenance Maintenance Replacing the Torsion Ribbon Assembly a 2. Grasp the pendulum bob here to stabilize it. If the torsion ribbon breaks, replace it as follows: 1. Remove the plates, and raise the locking mechanism using the locking screws until the pendulum arms are securely anchored (Figure 21a). 2. Grasp the pendulum bob near the bottom ribbon tab to stabilize it. 3.
Model No. AP-8215A Transporting and Storing Transporting and Storing 1. To prepare the Gravitational Torsion Balance for transporting or storing: a. Remove the front plate. b. Raise the locking mechanism to securely anchor the pendulum bob. c. Check to be sure that the torsion ribbon is hanging straight down the center of the tube. If it is not, lower the locking mechanisms, be sure the torsion wire is centered, and raise the locking mechanisms again.
Gravitational Torsion Balance Technical Support - Part number and revision (listed by month and year on the front cover); - Have the manual at hand to discuss your questions.
