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.
Faraday reasoned from Oersted's observation that electricity caused a magnetic needle to move, that magnetism could cause electricity. Faraday produced electricity to prove his point by creating a generator. Faraday also had the idea of electromagnetic fields, invisible lines of force.