What are the events that mark the beginning and the end of the third interphase period? What happens in the cell in this period?

The third interphase period is the G2. It starts with the end of DNA replication and ends with the beginning of the first period of the mitotic phase. On G2 the cell is growing too and the duplication of centrioles happens (only in cells that have these structures).

Thomson complete that the ray consisted of a stream of negatively charged particles that had been dislodged from atoms. These particles turn out to be known as electrons. Thomson went on to conclude that the mass of an electron is much less than that of a hydrogen atom.

Later, in 1909 Robert Millikan precisely determined the mass of an electron to be 1/1840 the mass of a hydrogen atom. He as well determined the electron's charge to be 1. This value was acknowledged as a fundamental unit of electrical charge.