The Einstein's first essay with the letter sent to his maternal uncle in 1895

The introduction to Einstein's essay on 'The Investigation of the State of Aether in Magnetic 

Fields,' which he wrote at the age of 15 or 16 and sent to his uncle Casar Koch with a covering 

letter. 

Reference: Volume 1

https://history-quantum-physics.blogspot.com/2018/12/the-golden-age-of-theoretical-physics.html