Garnett was born in
Portsea, Portsmouth, England in 1850, the son of William Garnett. In January 1863 he entered the
City of London School, where he was a pupil of Thomas Hall. In the May 1866 examination, he obtained the first Royal Exhibition, tenable at the
Royal School of Mines and College of Chemistry, and during the winter session, he studied under Dr.
Edward Frankland and Professor
John Tyndall, but in the following year, resigned the Exhibition and returned to the City of London School. He was head of the Maths Sixth Form when future prime minister Asquith was head of the Classical Sixth. In April 1869, he gained the Exhibition for Natural Science at
St John's College, Cambridge, and in July of the same year, the Beaufoy Mathematical Scholarship at the City of London School, and commenced residence at St. John's College in October. The following summer, he was first in the competition for Sir James Whitworth's Scholarship for Mechanical Engineering, and in 1871, was elected Foundation Scholar at St. John's College. In January 1873, he took his degree as
Fifth Wrangler, and shortly afterwards, on the opening of the
Cavendish Laboratory, he was appointed Demonstrator in Experimental Physics by Professor
James Clerk Maxwell. In November 1874, he was elected Fellow of St. John's College, and retained his fellowship till his marriage in 1879. ==Career==