The year 1997 will be remembered for at least two mega events, in which one is national when we celebrated nation's glorious 50th years of independence, while another is global when we celebrated 100th years of the discovery of the electron. For this we must give many many thanks to Prof. J.J.Thomson who discovered it in 1997. Let us come and see the journey of story of the Electron.

The term "atom" is derived from the Greek i.e not to be cut. Till the discovery of the electron, several speculations by the eminent philosophers and the scientists about the constituents of the matter and its indivisibility were around. These speculations helped the scientist to go forward in deciding the status of matter.The great Indian philosopher and follower of Vaisheshik Darshan "Maharishi Kanad" (much before Shankaracharya) argued that Kshiti, Jal, Agni, Vayu, Akash, Kal ,Diha, atma &Mon(i.e Earth,water,fire,space,time,soul and mind) are the constituents of matter in which five are physical .These five physical components combine to form the UNIVERS. Another Indian Philosophers"Charvak" did not believe in the existence of space because its knowledge is simply assumed.

The Vaisheshik Darshan also demonstrates that Kshiti, Jal, agni & Vayu are made from the different atoms. These atoms are physical neither created nor destroyed. Hence atoms are the smallest particle of the matter, which can be subdivided. In the late 5th century B.C,the Greek philosoher"Democritus" also used similar arguments about the indivisibility of the atom. "Empedocles"(490-430 B.C) who was not only a physicist and physician but also a good statesman, held that there are four indestructible and unchangeable elements Fire,Air,Water and earth eternally brought into union and parted from each other by two divine forces love and discord. Nothing new comes into and can come into being.. The only changes that can are thise juxtaposition of element with elements. The roman poet,"Luecrtieus"(98-55 B.C) argued that atoms are infinite in number and limited in their variation. The atoms, along with empty space, are the only eternal and immutable entities of which our physical world is made.

But in 17th century A.D, Renedescrates (1596-1650) pronounced that the opposite argument and held that there can not exit any atoms and parts of matter that are their own nature indivisible, because had rendered a particles small that it was not in power of any made to divide it.

Nineteenth Century A.D

in the beginning of the 19th century A.D , we found some positive aspects about the basic structure of the matter. In 1808,, The British Chemist, John Dalton(1766-1844) published his famous atomic theory, in which he stated that atom is the smallest indivisible part of any substance, so that they can not be divided further. He also showed various evidence in support of it in his famous book” New System Of Chemical Philosophy". Ina sequel volume he expressed the fundamental principle of the youngest of the science in these words--------- " I should apprehend that there are considerable number of what may be called elementary principles which can be metamorphosed one into another by any power, we can control. We ought, however, to avail ourselves of every means to reduce the number of bodies or principles of this appearances as much as possible and after all we may not know what elements are absolutely in decomposable and what are the refractory because we don't know the proper means of their reduction. all atoms of same kind, whether simple or compound must necessarily be conceived alike in the shape ,weight and every other particular".

These superb lines caused a flurry of hot debates between the physicist and chemists on the nature of atoms and molecules. The vital point of the debate among the chemists whether atoms were real objects or only memory devices for coding chemical regularities and the laws. In the view of the physicist, the whole issue was centered on the kinetic theory of heat, particularly the meaning of the second Law of thermodynamics.

These dichotomies between the physicist and the chemists came to a halt when Avogadro’s Hypothesis was put forward in 1811, according to which equal volumes of gases for equal temperature and pressure contains equal no. of molecules. This, in fact indicated the widespread resistance to the idea of molecular reality. Alexander Williamsons remark in his presidential address of 1869 to the "London Chemical Society' clearly showed that molecular reality met with less early resistance in physics than it did in chemistry. Physicist could already do things with molecules and atoms at a time where chemist could do most of the purposes, either to take them to be real or leave them as coding devices. Anyway until the last year of the 19th century most scientists believed in the reality of atoms, stating that these particles can't decompose further. The role of Bernoulli (1700-1782) and Radold Clausius(1822-1888) paper of 1887 entitled “on the kind of motion we call heat” was very helpful in bringing reality to the idea of molecules.

In 1897, Prof.J.J.Thomson discovered the fundamental particle earlier hypothetically named "ELECTRON" by G.J.Stoney in cathode rays experiment. This discovery not only exposed the limitation of the Dalton 's atomic theory but also brought new era in the world of new physics.Several important inventions in the experimental field preceded the discovery of the Electron. They include----- 

1.Higher Voltages: These resulted from the fruitful work done by the Heinrich Ruhmkorff (1803-1874)in designing an improved version of the induction coil.

2. Improved Vacuum his was achieved in 1855 by the German Scientists J.H.W.Geissler who invented a rotatory air pump of superior quality using which 99.9% of air would be pumped out of the discharged tube. With the help of this pump, Geissler was able to construct a sophisticated discharge tube, called Geissler Tube. This name was given by his friend and German physicist Julius Plicker(1801-1868).

3.Ionization Chambers : The parallel plate ionization chambers were developed in Cambridge during 1890's and were used by Rutherford and curies in the quantitative measurements of radioactivity.

4.Concave Spherical Grating : Henry Rowland developed this in 1890 at john Hopkins University .

5. Cloud Chambers : It was discovered by G.T.R Wilson, a student of Thomson in 1895, which helped Thomson in enabling him to measure the electric discharge.

CATHODE RAYS AND THE DISCOVERY OF THE ELECTRON

When Prof.J.J.Thomson applied a very high voltage above 50000 volts and very low pressure of about 10 to 5 mm of HG to a vacuum tube containing an element in gaseous form, a green fluorescence appeared around the anode9see fig 1.) . This happened because apparently some invisible rays emerge from the cathode and travel towards the anode with a speed equal to 3/5 th of the speed of the light. After striking the glass wall of the vacuum tube, a green fluorescence appears around the anode. Since they deflected towards the higher potentials of an applied electric field, they are supposed to be -vely charged particle called ELECTRONS. This stipulated that some of the particles come out from any gaseous element in the experiment, hence he further suggested that electrons is a Fundamental Particle.

By simultaneously deflecting the ray in both electric and magnetic fields, he was unable to determine their velocity and the ratio of m/e(i.e of mass to the electric charge) carried by these hypothetical particles, using the formula

Mass of the Electron, M E =e/(e/m) (1.6 x 10 -9)/1.76 x 10 7 = 9.1 x10 -31 Kg.

This mass can be considered as rest mass of the electron . That means this is the mass of the electron at rest or at velocities, which are small, compared with the velocity of light.

Mass of H atom:

It can be obtained by simply dividing the atomic weight of H by Avogadro number,N .Thus the mass of the H atom is

M H = (1.008)/6.023 X 10 23 = 1.67 X 10 –27 kg.

Comparing it with the mass of the electron, we find

M H/M E = (1.67X 10 -27 )/9.1 X10 -31 = 1835

Or M H = 1835 x M E

This equation states that the mass of the H atom is 1835 times heavier than the mass of the electron.

If the electron is regarded as purely electrical in origin, its mass will arise solely by virtue of its electric charge. This can be discussed on the basis of the electromagnetic theory.

The expression derived for the additional electromagnetic energy of the moving electron is

(1/2) x[(2/3)x( m e 2)/a],in o.k. units, where

m = Permeability of the medium

e= electronic charge

a= radius of the electron

v= velocity of the electron

if we compare it with the ordinary expression for the kinetic theory of the moving body, we find that the electron possess a mass [=(2/3)x( m e 2)] entirely due to its electric charge. This mass is known as electromagnetic mass. Thus we arrive at very interesting conclusion that the electron, the ultimate atom of the electricity, has no mass in material sense. This mass is purely electrical. It will be in justifiable lapse if we don’t mention other two names who may be considered as having contributed to the discovery of the electron ----(1) Emil Wiechart(1861-1928) & (2) Walter Kaufman(1871-1947).

Emil wiechart at the university of kornigsberg, privotdozent,did very good experiment on cathode rays.in course of giving lectrure on Jan7,1897 before “Kornigsberg Physival Economic Society”, he stated his conclusion about the cathode rays experiment that the mass of the moving particle turn out to be 2000 – 4000 times smaller than the mass of the atoms of the lightest known chemical atoms.

He was the first person who used the word “SUBATOMIC PARTICLE” in print, giving their mass,assuming the charge to be that of an electron.(“Als Ladung ist 1 Electron angenommen”)

Kaufman, on applying high voltage between electrodes inserted in a long evacuated glass tube, measured e/m ratio of the stream of particles constituting cathode rays and recognized anamolous nature of these results. He observed while submitting his memoir on cathode rays on Aug7,1897 that the smallness of the m/e may be due to the smallness of m or the largeness of e or due to the combination of these two. He went onto argue in favor of the smallness of m. Thus we have in the cathode rays, matter in ordinary gaseous state ,a state in which all matter -----is of on and the same kind this matter being the substance for which all the chemical elements are built up.

This observation was clear indication of the universality of constitution of these rays. Had he added one conjectured line to his paper (something like” If we assume e to be fundamental unit of charge, indentified in electrolysis then cathode rays must be considered to be new form of matter.”) he could have share the Nobel prize with the Thomson for the advancement made in 1897. Unfortunately this idea never developed in his mind and Thomson got the entire credit for the discovery of the electron.

Continuing on Thomson’s discovery, we observed that Thomson’s finest hour as experimentalists came in 1899 when he applied his method of measuring e/m ratio to the photo-electricity produced particles and reach the conclusion that these particles were also the Electrons.

“The value of e/m for the particles ejected from a metal by UV light is also the same as for the particles constituting the cathode rays, showing the identity of the Photo Electrons and the electrons in the cathode rays”.

He announced another experimental results for the value of charge of the electron which can be obtained by Wilson ’s cloud chamber experiment. Thomson determine the no. of charged particles by droplet counting (Milikan Oil Drop Experiment) and their overall charge by electrometric method and arrived at the remarkable results that the charge of the electron is about 4.8 x 10 -10 esu. This is the reason why he is said to be the discoverer of the electron.

He,Ipsofacto, presented a beautiful gift to the world of science which opened the floodgate to the other major scientific achievements and made 20 th century physics more productive.

(Note: This Article was accepted to publish in the journal of Vigyan Sanchar by Dept. Of Science &Technology ,Govt. Of Bihar In March 1997 )

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