Gauss' H-Variometer at the Helsinki Magnetic Observatory 1844-1912

Abstract

During the early decades of the 19th century variations of the geomagnetic field at geomagnetic observatories were observed visually only. The most widely used method to observe changes of variometer magnets was Poggendorff's mirror-and-scale system aided with a telescope. Readings were made typically 1 ... 12 times per hour. For observations of the horizontal field, Gauss' bifilar H-variometer was in use at geomagnetic observatories. At the Helsinki observatory, founded in 1838, Gauss' bifilar instrument was continuously in operation for more than 60 years. During that time about 1, 000, 000 readings of H were accumulated. Most of the data are still unreduced and are available in original observational notebooks only. The quality of the H-data, at least in the period 1844-1848 studied here, seems to be reliable. Comparisons between Helsinki H-results and the present day Nurmijarvi observatory data, reveal similar daily variation curves at both observatories. In long-term changes, with varying number of annual sunspots, the daily range of H follows the same linear relationship as H at Nurmijarvi. We believe that the Helsinki H-data series is reliable and homogeneous and thus suitable for detailed studies of changes of the geomagnetic horizontal field component in the mid 19th century.