voltage across the electrodes, or the strength of the magnetic field. By carrying out accurate measurements on the amount of deflections observed by the electrons on the electric field strength or magnetic field strength, Thomson was able to determine the value of e/m e as: = .758820 × C kg – ( . ) Where m e is the mass of the electron in kg and e is the magnitude of the charge on the electron in coulomb (C). Since electrons are negatively charged, the charge on electron is – e .
. . Charge on the Electron R.A. Millikan ( - ) devised a method known as oil drop experiment ( - ), to determine the charge on the electrons.
He found the charge on the electron to be – . × – C. The present accepted value of electrical charge is – .602176 × – C. The mass of the electron ( m e ) was determined by combining these results with Thomson’s value of e / m e ratio.
The apparatus to determine the charge to the mass ratio of electron . . Discovery of Protons and Neutrons Electrical discharge carried out in the modified cathode ray tube led to the discovery of canal rays carrying positively charged particles. The characteristics of these positively charged particles are listed below.
(i) Unlike cathode rays, mass of positively charged particles depends upon the nature of gas present in the cathode ray tube. These are simply the positively charged gaseous ions. (ii) The charge to mass ratio of the particles depends on the gas from which these originate. (iii) Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
(iv) 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 . Later, a need was felt for the presence of electrically neutral particle as one of the