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The Cavendish Gravitational Balance is part of the advanced physics products offered by Spectrum Scientifics. Built by United Scientific, these advanced physics devices are built on demand and are excellent for University or Advanced High School physics classes.

2471The Cavendish Gravitational Balance is used to help students and teachers measure the Universal Gravitational Constant, also known as “‘G”. “G” as some of you may remember is rather small since gravity is a very weak force. The exact measurement is 6.67384 × 10-11 m3 kg-1 s-2. This has been accurately determined by a couple of centuries of experimentation and math. The Cavendish Gravitational Balance will allow students to measure ‘G’ to within 15% accuracy.

15% may not sound very accurate but once again you must remember how small ‘G’ actually is. It is so small that the measurement must use lead balls to prevent any effects of magnetic interference (such as say from the Earth’s magnetic field) as well as reduce static.

The base experiment of the Cavendish Balance is to recreate the Cavendish experiment. Cavendish’s experiment placed two small (under 1 kg) lead balls placed on a six foot long torsion bar. Two much larger 150 kg lead balls were placed within 9 inches of the smaller balls and held by their own suspension system. The mutual gravitational attraction caused the torsion bar to twist and from that measurement (and known the properties of the torsion wire, etc) ‘G’ was determined. Despite doing this in the late 18th century Cavendish was accurate to within 1%.

An experiment this large, needless to say, is not very practical. So to replicate it in the small class lab the Cavendish Gravitation Balance uses two 1.5kg lead balls (the large black balls you can see in the picture) resting on aluminum stands that can swivel. The Pivoting masses are held by the torsion system and only have a mass of 20g.

The Cavendish Gravitational Balance actually has three methods of measuring ‘G’

First is to derive the value by measuring the angular acceleration of the balance over a period of 90 seconds after the large masses are moved.  This is fast, but the least accurate method of measurement.

The Second is known as the equilibrium displacement. This is done by measuring the change from the rest position after the large lead spheres are swiveled into their new position. This requires 10 minutes to measure, and that is after the system has had to dampen down for an hour.

The final system is known as the pendulum system. Here a mirror is used is an optical lever. This is a bit hard to explain.

 

As one might guess, this system is actually quite sensitive. The casing is 25mm thick with glass windows to prevent drafts from effecting the system. The base rests on three leveling feet, while the torsion system uses damping oil to help settle the the central system. This is needed is even a slight vibration in the system can throw off the results. The oil reduces the amount of time needed to dampen the system.  The oil is provided.

The Cavendish Gravitational Balance requires several other common lab items to work. A digital balance (2000g) an vernier caliper are needed for accurate measurement of the lead balls. A laser pointer is needed for the optical lever method, and s stop watch is needed for timing the 90 seconds for the first measurement method.

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