R.A. Millikan (1868-1953) devised a method known as oil drop experiment (1906-14), to determine the charge on the electrons. He found that the charge on the electron to be

– 1.6 × 10^–19 C. The present accepted value of

electrical charge is – 1.6022 × 10^–19 C.

 The mass of the electron (m_e) was determined by combining these results with Thomson’s value of e/m_e ratio.

Me=ee/mₑ =1.6022 x 10¯¹⁹ c1.758820 x 1011c kg¯¹

 Discovery of Protons and Neutrons Electrical discharge carried out in the modified cathode ray tube led to the discovery of particles carrying positive charge, also known as canal rays. The characteristics of these positively charged particles are listed below.

1. Unlike cathode rays, the positively charged particles depend upon the nature of gas present in the cathode ray tube. These are simply the positively charged gaseous ions.
2. The charge to mass ratio of the particles is found to depend on the gas from which these originate.
3. Some of the positively charged particles carry a multiple of the fundamental unit

of electrical charge.

4. The behaviour of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode rays.

The smallest and lightest positive ion was obtained from hydrogen and was called proton. This positively charged particle was characterised in 1919. Later, a need was felt for the presence of electrically neutral particle as one of the constituent of atom. These particles were discovered by Chadwick (1932) by bombarding a thin sheet of beryllium by α-particles. When electrically neutral particles having a mass slightly greater than that of the protons was emitted. He named these particles as neutrons.