hit by them. Television picture tubes are cathode ray tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials. (iii) In the absence of electrical or magnetic field, these rays travel in straight lines (Fig. .
). (iv) In the presence of electrical or magnetic field, the behaviour of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons. (v) The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube. Thus, we can conclude that electrons are basic constituent of all the atoms.
. . Charge to Mass Ratio of Electron In , British physicist J.J. Thomson measured the ratio of electrical charge ( e ) to the mass of electron ( m e ) by using cathode ray tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons (Fig.
. ). When only electric field is applied, the electrons deviate from their path and hit the cathode ray tube at point A (Fig. .
). Similarly when only magnetic field is applied, electron strikes the cathode ray tube at point C. By carefully balancing the electrical and magnetic field strength, it is possible to bring back the electron to the path which is followed in the absence of electric or magnetic field and they hit the screen at point B. Thomson argued that the amount of deviation of the particles from their path in the presence of electrical or magnetic field depends upon: (i) the magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater is the interaction with the electric or magnetic field and thus greater is the deflection.
(ii) the mass of the particle — lighter the particle, greater the deflection. (iii) the strength of the electrical or magnetic field — the deflection of electrons from its original path increases with the increase in the