Answer: The charge of an electron
Step-by-step explanation:
The charge on electrons was determined by the Robert Millikan and Harvy Fletcher from their experiment named Oil drop experiment.
The charge on the electrons is

Whereas Neil Bohr in his structure of atom explained that:
- Electrons are moving around the nucleus in a discrete circular paths known as orbits or shells.
- These orbitals or shells have fixed value of energy.
- Emission spectra of hydrogen.