His discoveries included his investigations of electricity. Franklin proposed that "vitreous" and "resinous" electricity were not different types of "electrical fluid" (as electricity was called then), but the same electrical fluid under different pressures. He was the first to label them as positive and negative respectively, and he was the first to discover the principle of conservation of charge. In 1750, he published a proposal for an experiment to prove that lightning is electricity by flying a kite in a storm that appeared capable of becoming a lightning storm. On May 10, 1752, Thomas-François Dalibard of France conducted Franklin's experiment (using a 40-foot (12 m)-tall iron rod instead of a kite) and extracted electrical sparks from a cloud. On June 15, Franklin may have possibly conducted his famous kite experiment in Philadelphia and also successfully extracted sparks from a cloud, although there are theories that suggest he never performed the experiment. Franklin's experiment was not written up until Joseph Priestley's 1767 History and Present Status of Electricity; the evidence shows that Franklin was insulated (not in a conducting path, since he would have been in danger of electrocution in the event of a lightning strike). Others, such as Prof. Georg Wilhelm Richmann of Saint Petersburg, Russia, were electrocuted during the months following Franklin's experiment. In his writings, Franklin indicates that he was aware of the dangers and offered alternative ways to demonstrate that lightning was electrical, as shown by his use of the concept of electrical ground. If Franklin did perform this experiment, he did not do it in the way that is often described, flying the kite and waiting to be struck by lightning, as it would have been fatal. Instead, he used the kite to collect some electric charge from a storm cloud, which implied that lightning was electrical.
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