Franklin's Lightning Conductor

It was while living in Philadelphia that Benjamin Franklin, the 18th Century statesman and natural philosopher, became convinced that electricity and lightning were the same phenomenon. The Leyden jar made it practicable to prove this supposition and in 1752 he began his celebrated experiments with kites. He succeeded in flying an ordinary silk kite on a light wooden cross during a thunderstorm. The centre of the cross was provided with a sharp iron spike and below was a key which was in contact with a Leyden jar. To his delight the lightning caused a flow of current which was clearly indicated by the jar. After many such experiments, Franklin concluded that buildings might be protected against lightning in the same way by a pointed wire standing erect above the roof with the lower end buried in the ground.

In 1769 Franklin visited England and lightning-conductors to his specification were erected on St. Paul's Cathedral. These conductors were of iron, 4 in. x 1/2 in. in section. Their electrical resistance was so high that when the Cathedral was struck by lightning in 1772, portions of one conductor were made red-hot by the passage of the discharge.

As a consequence copper came to be used for lightning conductors, an application for which it has been virtually unchallenged ever since. In 1811 copper wire was employed to protect ships' masts; prior to this it had been customary for vessels to carry copper chains which had to be hoisted to the masthead whenever a storm appeared to be imminent; contact with the sea was made by means of a connexion outside the hull. Hoisting the chains was not always carried out sufficiently rapidly, so that the ship was often damaged before they could be got into position. It was therefore decided to fix permanent conductors, and at the suggestion of Benjamin Cook copper strips were run down each of the masts and through the hull to the ship's bottom which, in the case of naval vessels, was usually sheathed in the same metal. Through sheer ignorance, the remedy was sometimes almost as bad as the cause, for it is recorded that on one man-o' war the conductor actually passed through the powder magazine. (25)

(25) STURGEON, W. Lectures on Electricity (1841), p. 207.