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.
Many natural philosophers and mathematicians like Robert Boyle, Otto von Guericke, Stephen Gray and C.F. Du Fay investigated the phenomenon of electricity. In the 18th century, Benjamin Franklin is supposed to have first proved conclusively that lightning was indeed electricity, through some kite experiments.