The amplitude (dD) of the diurnal variation of the magnetic Declination [or the equivalent East Component, Y, of the geomagnetic field] is a reliable indicator of solar Far UltraViolet radiation (FUV) and of its proxy, the sunspot Number, R [as was known already to Rudolf Wolf ~160 years ago]. FUV creates and maintains the E-layer of the ionosphere, determining the conductivity, and hence the strength of the current causing the diurnal variation. We show how the changes of sunspot observers are faithfully reflected as discontinuities in the relationship between dD and R. On the whole, sunspot numbers before ~1945 should be adjusted upwards by 20% and before Wolf's death by another 30%, with the net result that 20th century solar activity does not seem significantly larger than that for the 19th. Support for the above conclusion comes from a comparison with the Greenwich Sunspot Areas which indicate a 17.5% increase of Rz coincident with Max Waldmeier's tenure as observer in Zurich. The Mount Wilson Ca II K-line index recently derived by Bertello also indicate a 20% increase in Rz ~1945. From 1934 researchers have measured the foF2 frequency of the ionospheric F-layer and found that this measure has a very strong correlation with the sunspot number. It was later noted that the correlation changed ~1945 and has been different since, consistent with the same ~20 percent artificial increase in Rz introduced by Waldmeier. Each of these indications by itself would be just a curious discrepancy for which an ad-hoc explanation might be concocted. Collectively, however, they provide strong evidence for quantifiable inhomogeneities in the sunspot series coincident with changes of observer, and we propose that the series be corrected accordingly.
