Who experimentally demonstrated the quantisation of electrical charge?

Millikan

Robert Millikan, along with Harvey Fletcher, experimentally demonstrated the quantization of electrical charge in 1909 through the famous "oil drop experiment". This experiment measured the charge on individual oil droplets by balancing their weight with the electrical force exerted by a known electric field. The results showed that the charges on the droplets were all multiples of a fundamental unit, which we now call the elementary charge (e).