A correlation is a single numerical value that is used to describe the relationship between two variables. All correlations have strength and direction. When two variables have a positive correlation, that means the two variables move in the same direction. The closer a positive correlation is to 1, the stronger it is.
William Gilbert discovered electricity, not Nikola Tesla. Nikola Tesla invented things. not discovered electricity. Again, William Gilbert discovered it and named it. He did this in the 1600s. It wasn't Nikola Tesla.
The phenomena of electricity had been known by the ancient Greeks. Benjamin Franklin had displayed the relationship between lightning and electricity. But, it was the invention of the Electric Dynamo by Michael Faraday which really opened up the way to the practical use of electricity.