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structure of atom 31 material of electrodes and the nature of (iii) the strength of the electrical or magnetic the gas present in the cathode ray tube. field — the deflection of electrons from its original path increases with the increase Thus, we can conclude that electrons are basic constituent of all the atoms. in the voltage across the electrodes, or the strength of the magnetic field. 2.1.2 C harge to Mass Ratio of Electron By carrying out accurate measurements In 1897, British physicist J.J. Thomson on the amount of deflections observed by measured the ratio of electrical charge (e) to the mass of electron (m ) by using cathode the electrons on the electric field strength or e magnetic field strength, Thomson was able to ray tube and applying electrical and magnetic determine the value of e/m ae: fieldperpendicular to each other as well as to the path of electrons (Fig. 2.2). When only = 1.758820 × 10 11C kg –1 (2.1) electricfield is applied, the electrons deviate from their path and hit the cathode ray tube Where m e is the mass of the electron in kg at point A (Fig. 2.2). Similarly when only and e is the magnitude of the charge on the magnetic field is applied, electron strikes electron in coulomb (C). Since electrons are the cathode ray tube at point C. By carefully negatively charged, the charge on electron balancing the electrical and magnetic field is –e. strength, it is possible to bring back the 2.1.3 Charge on the Electron electron to the path which is followed in the absence of electric or magnetic field and they R.A. Millikan (1868-1953) devised a method hit the screen at point B. Thomson argued known as oil drop experiment (1906-14), to determine the charge on the electrons. that the amount of deviation of the particles from their path in the presence of electrical He found the charge on the electron to be or magnetic field depends upon: – 1.6 × 10–19C. The present accepted value of electrical charge is – 1.602176 × 10 –19C. The (i) the magnitude of the negative charge on mass of the electron (m ) was determined by the particle, greater the magnitude of e the charge on the particle, greater is the combining these results with Thomson’s value of e/m eatio. interaction with the electric or magnetic field and thus greater is the deflection. (ii) the mass of the particle — lighter the –31 particle, greater the deflection. = 9.1094×10 kg (2.2) Fig. 2.2  The apparatus to determine the charge to the mass ratio of electron Reprint 2026-27