Rosenhain then did three years research work with Professor James Alfred Ewing at St John's College, Cambridge.
[3] On the advice of his professor he took up the microscopic examination of metals, and spent some time at the Royal Mint studying the technique of his new work.
In 1906 Rosenhain became the first superintendent of the department of metallurgy and metallurgical chemistry at the National Physical Laboratory, he held this position until 1931.
He appointed the NPL's first female scientific staff members in 1915, Marie Laura Violet Gayler and Isabel Hadfield.
[4] Rosenhain published a large number of papers and addresses, and his highly trained staff also did much writing, covering the whole field of physical metallurgy, ferrous and non-ferrous.
In 1927 he was appointed British delegate on the permanent committee of the International Association for Testing Materials, and was elected its president at the Zurich congress held in 1931.