A Few years back, we didn’t know the structure of atom; in fact John Dalton considered it to be as hardand dense and as the smallest and indivisible part of matter. But that fact didn’t remain true for long and a new concept in the field of atomic structure came in to existence.
The phenomenon of emission of positively and negatively charged particles by radioactive elements and by gasses when electricity passed through theses gages at very low pressures made one point clear to scientists that atom is not the indivisible part of matter and consists of much smaller and fundamental particles. The rays emitted by cathode of a discharge tube under very low pressures are known as cathode rays. Then its properties were studied and were found as follows:-
- Cathode rays travel in a straight line and are always emitted at right angle to the cathode.
- These rays constitute material particles as these can produce mechanical motion in a small paddle wheel kept in its way.
- Its constituting particles are electrically charged as deflected under electrical and magnetic field and direction of deflection under electric field indicated that these are negatively charged particles.
The negatively charged particles constituting the cathode rays emitted by cathode surface are known as electrons.
J.J Thompson was awarded the physics Nobel Prize in 1906 for his contribution in the field of atomic structure. He determined the e/m ratio of electron which he later proved it to same irrespective of the gas taken and hence proved that electron is the fundamental particle constituting atom.
He subjected the beam of electrons to the perpendicular applied magnetic field. The beam of electrons began to move in a circular pat of radius r under the influence of magnetic field Bev.
Here, B is the strength of magnetic, e is the charge on electron and v is electron’s velocity.
So, the centrifugal force acting on beam of electrons is equal to the magnetic field applied:-
Bev=mv2/r
Or e/m=v/(rB)……………………….(1)
Then he applied the electric field of strength E and each electron experienced a force Ee. The two applied were so adjusted as to make the beam of electron pass without any deflection under their influence i.e. the two applied fields were equal and opposite.
Bev=Ee
Or v=E/B.
Now putting this value of v in equation (1), we get:-
e/m=E/(rB2)
Values of E and B are known, value of r is obtained by the deflection under magnetic field only on the fluorescent screen. So by putting all these values in above equation, the value of e/m was calculated and came out to be same for all the gases taken.