Penny and Cotton (AKA Guinea and Feather), 1C20.10

Location:

• Cabinet: [:MechanicsCabinet:Mechanic (ME)]

• Bay: C3

• Shelf: Cubby

attachment:PennyCotton-01-400.jpg

Description:

A glass tube contains a piece of cotton ball and a penny. It is fitted with a valve on one end so that it may be attached to a vacuum pump.

Important Setup Notes:

• N/A

Setup:

1. Make a Setup check list.
2. ...

Cautions, Warnings, or Safety Concerns:

1. The tube is fragile. Handle with care.

Demonstration:

Show that all objects would fall at the same rate were it not for air resistance. Invert the tube once, and then again when evacuated.

A glass tube at least a meter in length is equipped with a valve and nozzle through which the tube can be evacuated with a vacuum pump. Such a tube is called a guinea and feather tube because those were the objects traditionally used when the demonstration was performed in England centuries ago. The guinea was an English gold coin issued from 1663 to 1813 and valued at 21 shillings. In this tub, a modern coin and a ball of cotton are used. Our penny is so old that the edge of the penny is distorted for the years of use.

One can introduce this demonstration by asking the audience which falls faster, the cotton or the penny. Whichever answer is given is either right or wrong depending upon whether the experiment is performed in the air or in a vacuum. One can point out that in science there are often opposing theories, and in such cases, experiments are required to determine which theories are correct. Actually, an experiment usually cannot prove that a theory is correct, only that it is incorrect.

Places both objects at one end of the tube at atmospheric pressure. Then rapidly inverts the tube, yielding the expected result. Then evacuated the tube by use of a ruffing pump, and the demonstration is then repeated. Note now that both objects fall at the same rate when the tube is evacuated. Of course, the part of the demonstration not requiring a vacuum can be done without use of the tube with the demonstrator standing on the lecture table for a bit of extra drama.

If a vacuum pump is not available, an alternate form of the demonstration can be done using a heavy book and a sheet of paper (smaller than the book)[1]. The two are first dropped side by side. The book will fall much faster. The paper is then placed flat on top of the book and the two are released together. They will fall at the same rate because the book eliminates the air resistance that the paper would otherwise experience.

References:

1. [http://sprott.physics.wisc.edu/demobook/chapter1.htm Prof. Sprott's Notes]

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