At the age of 12, I think, Gustaf derived the rule for computing the currents and voltages in an electric circuitry.

Worked on the problems of thermodynamics - black body radiation - and along with Bunsen (of Bunsen torch fame) engineered the optical spectroscope and spectrograph for investigation of optical spectra and, thus, recognition of chemical elements and compounds.

Instructed his student Max Planck to further study black body radiation. That resulted (1900) in Planck's constant ("Google" it) and the beginning of quantum physics that since then has been changing our view and knowledge of the world.