1L10.30 - Cavendish Balance

Code Number:
1L10.30
Demo Title:
Cavendish Balance
Condition:
Good
Principle:
Measurement of Gravitational Constant 'G'
Area of Study:
Mechanics

Disclaimer

Equipment:
Cavendish Balance, Laser, Rubber Mat, Lecture Room Stand, 2 Meter Scale, Computerized Cavendish, Differential Capacitor Unit, KIS Interface, Multimeter, and Laptop.
Procedure:

The computerized Cavendish is designed to be worked with an interface.  Results can be obtained within a matter of minutes instead of a 1 hour class period.  Usually 15 minutes is enough time to stabilize the apparatus once the torsion arm has been released.  Once the balls are in position wait several minutes before turning on the interface.  When the balls are moved to their new position again wait several minutes before turning on the interface.  The multimeter allows you to zero the apparatus when it is at equilibrium and also allows you to monitor the voltage so that you can set the proper scale.

In Lecture Room 1 you may set the balance up on the sunken piling and special stand that is available and set the demo scale on the opposite wall, however this is very time consuming and almost always unnecessary.  Take it into the lecture room at least 1 hr ahead of demonstration time and prepare it for actual use.  Make sure that nothing disturbs the table after this time so that the balance has time to reach a relative equilibrium.  For actual demonstration all the teacher should have to do is turn on the laser and mark on the wall where the starting point is.  Then change the masses and wait until the end of class for equilibrium and the marking of the second point.  Distance apart should be about 18 inches but this also depends upon how close the laser is to the balance.

References:
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Berlin Group - Interactive Screen Exp. - Cavendish Experiment
Hume and Ivey - Universal Gravitation
Harvard Cavendish Webpage
PDF icon Cavendish Balance Manual and Torsion Fibre Replacement

Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.