School of Physics Lecture Demonstration Manual

Fb-8 Mercury Amoeba

Published: Friday 10 March 2006 - Updated: Tuesday 29 March 2011

Aim

To demonstrate that surface tension is influenced by surface charge density.

Apparatus

Diagrams

Click on pictures to enlarge.

Mercury Amoeba 2

Mecury Amoeba 1

Description

The success of this demonstration depends on cleanliness, correct concentration and positioning of the short circuiting nail. A suitable concentration is 5 ml of dilute Sulphuric acid in 500ml of water with a few crystals of Potassium Dichromate. Correct positioning of the nail will set the mercury into oscillation as shown below.

Electrical charges concentrate at surfaces where their repulsive forces on each other subtracts from the surface tension. The mercury in the dilute sulphuric acid together with the Potassium Dichromate forms a chemical battery when the iron nail is brought in as a second electrode. When the two metals touch the battery is short circuited and the charge density on the mercury surface is reduced. The mercury drop pulls up due to increased surface tension. With just the right position for the nail the mercury will set itself into oscillation, repeatedly short circuiting the electrical charges that build up when the mercury is not touching the nail.

Safety notes

Electrical Safety 

Chemical Safety 

Sharps Safety 

Personal Protection 

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