Why is charge of the electron negative?

How did scientists figure out that the charge of the electron was indeed negative? I know how the cathode ray tube experiment works, but how did Thompson know that the plate that the cathode ray beam was attracted to was positive, meaning the cathode ray was negative? What is the history behind positive and negative charges leading up to the cathode ray experiment. Ben Franklin postulated that a body with excess electricity was positive as in a surplus of electricity, and that surplus flows from positive to negative. But how and when was it discovered that electricity flows from negative to positive?