School of Physics Lecture Demonstration Manual

He-2 The Maxwell Demon Bottle

Published: Monday 13 March 2006 - Updated: Monday 28 March 2011


To demonstrate the statistical nature of irreversible processes.


The Maxwell Demon bottle


Click on pictures to enlarge.

Maxwell Demon Bottle Pic

Maxwell Demon Bottle Diagram


The Maxwell Demon demonstrator consists of a sealed flask containing 5 white and 5 black spheres. The demonstration begins with the balls in the stem of the flask in the unmixed condition. Inverting the flask mixes the balls and a discussion of the probability of the balls unmixing on inverting the flask again can follow. With 5 black and 5 white balls the odds against reproducing the initial array are 252 to 1.

Maxwell's Demon can be introduced and his task illustrated in the following manner; rotate the flask in a stem up position to impart a swirling motion to the spheres. Turn the flask to the stem down position and slowly reduce the rotation rate and the balls will spiral back into the neck. With a little practice the initial array is restored.

The "demonability" resides in the fact that the black spheres are constructed so as to rapidly lose momentum (see diagram).

Safety notes

Sharps Safety 


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