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.
Heinrich Hertz discovered photoelectric effect in 1887. Although electrons were not discovered yet, Hertz observed that electric currents were produced upon the incidence of ultraviolet light. By the beginning of the 20th century, physicists confirmed that: