Positive rays – (Discovery of Proton) :
u The first experiment that lead to the discovery of the +ve particle was conducted by ‘Goldstein‘.
u He used a perforated cathode in the modified cathode ray tube.
u It was observed that when a high potential difference was applied b/w the electrodes, not only cathode rays were produced but also a new type of rays were produced simultaneously from anode moving towards cathode and passed through the holes or canals of the cathode. These rays were termed canal rays since these passed through the canals of the cathode.These were also named anode rays as these originated from anode.
u When the properties of these rays were studied by Thomson, he observed that these rays consisted of positively charged particles and named them as positive rays.
u The following characteristics of the positive rays we recognised :
(i) The rays travel in straight lines and cast a shadow of the object placed in their path.
(ii) Like cathode rays, these rays also rotate the wheel placed in their path and also have heating effect. Thus, the rays passess K.E. i.e. mass particles are present.
(iii) The rays are deflected by electric and magnetic fields towards the negatively charged plate showing thereby that these rays carry +ve charge.
(iv) The rays produce flashes of light on ZnS screen
(v) These rays can pass through thin metal foil.
(vi) These rays can produce ionisation in gases.
(vii) Positive particles in these rays have e/m value much smaller than that of e-. For a small value of e/m, it is definite that positive particles possess high mass.
(viii) e/m value is dependent on the nature of the gas taken in the discharge tube, i.e. +ve particles are different in different gases.
u Accurate measurements of the charge and the mass of the particles in the discharge tube containing hydrogen, the lightest of all gases, were made by J.J. Thomson in 1906. These particles were found to have the e/m value as +9.579 ´ 104 coulomb/g. This was the maximum value of e/m observed for any +ve particle.
u It was thus assumed that the positive particle given by the hydrogen represents a fundamental particle of +ve charge. This particle was named proton by Rutherford in 1911. Its charge was found to be equal in magnitude but opposite in sign to that of electron.
Thus
charge on proton = + 1.602 ´ 10-19 columb i.e. one unit +ve charge
u The mass of the proton, thus can be calculated.
Mass of the proton = = 1.672 ´ 10–24 g = 1.672 ´ 10–27 kg
Mass of proton in amu = = 1.00757 amu.