Bartels, J. (1934), "Twenty-Seven Day Recurrences in Terrestrial-Magnetic and Solar Activity, 1923-1933", Terrestrial Magnetism and Atmospheric Electricity, 39 (3): 201—202a, Bibcode:1934TeMAE..39..201B, doi:10.1029/TE039i003p00201
Bartels, J. (1934), "Twenty-Seven Day Recurrences in Terrestrial-Magnetic and Solar Activity, 1923-1933", Terrestrial Magnetism and Atmospheric Electricity, 39 (3): 201—202a, Bibcode:1934TeMAE..39..201B, doi:10.1029/TE039i003p00201