The carbon arc

Carbon is an amazing chemical element. Its atoms can form bonds with themselves in an amazing number of different ways forming solids such as diamond, graphite and even football shaped molecules (C60) ! It can also combine with many of the other elements to form much of the complex world around us including life.

click here for details of C60

It also forms bonds that can withstand incredible high temperatures and spectroscopic measurements have shown that molecules of C2 can be found in the surface of stars with temperatures higher than 3000 C !

A very important form of pure carbon is called graphite. It is a very versatile and industrially important material. The combination of graphite's mechanical properties (it can be cut and machined to almost any shape), its electrical properties (it is a very good conductor) and its high temperature durability (starts to evaporate 2000-3000C) makes it unique.

200 years ago people first started to use graphite with electricity to make light and heat - it is called the carbon arc. It is very easy to demonstrate a carbon arc. Simply wire two carbon rods (also called graphite rods) via heavy gauge cables to a 12V car battery and then touch the two carbon rod ends together - a brilliant white light arc is produced. The arc is very hot - perhaps over 2000 C. There is not only thermal energy generated but also much light, and the simple carbon arc was used to make war time search lights as well as the bright light needed for projectors in the early days of cinema.

In the Rough Science series we used the high temperatures produced by the arc to melt the gold we had purified. We got the carbon rods for the experment by dismantling torch batteries.

Note: care must be taken dismantling batteries as the chemicals are potentially hazardous. Obviously care must also be taken with the high temperatures of the arc.

THE CREATIVE SCIENCE CENTRE

Dr Jonathan Hare, Physics Dept., The University of Sussex
Brighton, East Sussex. BN1 9QJ

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