The heaviest particle among the following is:

The ratio of specific charge of a proton to that of an $\alpha -$particle is:

In Millikan's experiment,static electric charge on the oil drops has been obtained by shining $X$-rays. If the static electric charge on the oil drop is $-1.282 \times 10^{-18} \ C,$ calculate the number of electrons present on it.

Fill in the blanks:
$(i)$ $R. A. Millikan$ developed the ......... method to determine the charge of an electron.
$(ii)$ The positively charged particles in a modified cathode ray tube are called ............ .
$(iii)$ The smallest and lightest positive ion obtained from hydrogen is ........... .
$(iv)$ In the year $1932$,$Chadwick$ discovered ........... .

$A$ particle having the same charge and $200$ times greater mass than that of an electron is:

The fact that the atom is divisible was determined from ............ .