Lightning conductor, c 1900.
© Science Museum / Science & Society Picture Library
This conductor directs lightning through a copper strip to the earth, leaving buildings safe. Lightning is a high-voltage electrical discharge between two charged rainclouds or between a cloud and the ground, caused by a build-up of opposed electrical charges. Currents of 20,000 amperes and temperatures of 30,000 degres Celsius (54,000 Fahrenheit) commonly occur, hence the need for lightning conductors.