Вставьте артикль где необходимо.When Edison was … boy of fifteen he worked as … telegraph operator. He had to be on …

When Edison was a boy of fifteen he worked as a telegraph operator. He had to be on duty from 7 p.m. to 7 a.m. and give a signal every hour to prove that he did not sleep. The signals were made with astonishing exactness. One night an inspector arrived and saw Edison sleeping on a chair. He was about to shake him when he caught sight of a mechanism on the table near the telegraph instrument. He waited to see what would happen. When the hand of the clock pointed to four, the instrument got busy and one level threw open the key while the other sent the signal over the wire. The inspector seized the sleeping boy, roused him and fired him. That is why the first of Edison’s numerous inventions was never patented.