 In 1914 , Franck teamed up with Hertz to perform an experiment to investigate fluorescence . They designed a vacuum tube for studying energetic electrons that flew through a thin vapour of mercury atoms . They discovered that when an electron collided with a mercury atom it could lose only a specific quantity ( 4 @.@ 9 electron volts ) of its kinetic energy before flying away . A faster electron does not decelerate completely after a collision , but loses precisely the same amount of its kinetic energy . Slower electrons just bounce off mercury atoms without losing any significant speed or kinetic energy . 
