Millikan's experiment to determine the electron charge
   In 1900 Millikan performed several experiments to determine the charge of the electron using the apparatus shown in the figure.

   X-rays ionize the air between two metallic plates. As a result of this, some electrons, which are now free, stick with oil droplets, which are sprayed in the region between the two plates. The oil droplets will be negatively charged.

   In the absence of an electric field between the two plates, the oil droplets will move downwards under the effect of gravity. This can be observed using a microscope.

   Connecting a potential difference V across the two plates, an electric field is generated between them (E = V/d where d is the distance between the two plates) in the direction shown in the figure. Then the electric field is adjusted such that the electric force (F_E = eE) is equal to the force due to gravity  (F_G = mg).

   Therefore, eE = mg  ---> e = mg/E

   By knowing the mass of the oil droplet m, the acceleration due to gravity g and the electric field intensity E at which balance takes place, we can calculate the charge of the electron. Experimentally it was found that the calculated charge is not always the same. It is multiples of a certain value which is 1.6019x10^-19 coulomb.

   This is the charge of electron measured by another method.

Chadwick's experiment to discover the neutrons
   In 1935, Chadwick was awarded the Nobel prize for an experiment done in 1932.

   In this experiment, alpha particles are projected towards beryllium target. The emitted particles are allowed to fall on paraffin wax, which in turn releases another type of particles. The study of the properties of such particles showed that they are protons. From the energy calculations, Chadwick showed that the particles released from beryllium, as a result of the incidence of alpha particles on it, are uncharged and have the same mass as protons. He called them neutrons.

   The reaction of the experiment can be written in the form:
        ⁴₂He + ⁹₄Be -----> ¹²₆C + ¹₀n

    The number written at the bottom left is the atomic number and that written at the top left is the mass number. Alpha particle is the helium nucleus of atomic number 2 and mass number 4, beryllium is of atomic number 4 and mass number 9, carbon is of atomic number 6 and mass number 12 and the neutron is of atomic number 0 and mass number 1.

   In nuclear reactions, the atomic and mass numbers are conserved. That is, the sum of atomic numbers on the both sides of the equation are the same and so for mass numbers.

   These particles (neutrons), resulting from the bombardment of beryllium with alpha particles, on falling on paraffin wax, interact elastically with hydrogen atoms. As a result of this, neutrons stop and protons (hydrogen nuclei) are ejected from paraffin.

   There is a clear difference between protons and neutrons. Neutrons are uncharged for that they have high penetration power on traveling through a certain medium. This is not the case of protons.

   The discovery of neutrons enabled us to understand the structure of the nucleus. It consists of protons and neutrons. The number of protons is called the atomic number (z) and the summation of the number of protons and neutrons is called the mass number (A). 

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