The Annalen also served as a source of modest additional income for Einstein, who wrote more than twenty reports for its Beiblätter – mainly on the theory of heat – thus demonstrating an impressive mastery of the contemporary literature. This activity started in 1905. and probably resulted from his earlier publications in the Annalen in this field. Going by his publications between 1900 and early 1905, one would conclude that Einstein's specialty was thermodynamics.