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Using several lines of evidence we show that the scale values of thegeomagnetic variometers operating in Helsinki in the 19th century were notconstant throughout the years of operation 1844-1897. Specifically, the adoptedscale value of the Horizontal Force variometer appears to be too low by ~30%during the years 1866-1874.5 and the adopted scale value of the Declinationvariometer appears to be too low by a factor of ~2 during the interval1885.8-1887.5. Reconstructing the Heliospheric Magnetic Field strength fromgeomagnetic data has reached a stage where a reliable reconstruction ispossible using even just a single geomagnetic data set of hourly or dailyvalues. Before such reconstructions can be accepted as reliable, the underlyingdata must be calibrated correctly. It is thus mandatory that the Helsinki databe corrected. Such correction has been satisfactorily carried out and the HMFstrength is now well constrained back to 1845.

Errors in Scale Values for Magnetic Elements for Helsinki