The present study applies classical techniques of time series analysis to the separation of the external and internal signals from the geomagnetic field. We first differentiate the data with respect to time in order to get rid of the large secular variation over the long time span considered (up to 100 years). Next we filter the resulting data to eliminate short-period components of the magnetic signals. We then proceed to identify the external signal by comparing the long-term variations of the processed components of the geomagnetic field, as measured at different European observatories, with the long-term variations of geomagnetic indices devised to characterize the fields from external sources. Finally, we remove the identified external signal from the geomagnetic field series and get a satisfactory estimate of the internal signal. This procedure is facilitated by the different behaviors of the two signals over the considered time span. The external signal is inherently of recurrent nature linked to solar-terrestrial interaction; it oscillates around zero with a maximum amplitude of about 5 nT/yr. The internal signal, on the other hand, displays the characteristics of a secular trend, combining sustained monotonous behavior over periods of several decades with sudden slope variations and reversals; the total range of this internal secular signal is of the order of 50 nT/yr, far larger than any external contribution. Using the 1883--1983 magnetic series at Chambon-la-Foreˆt, the 1890--1983 series at Niemegk, and shorter series at the United Kingdom observatories of Eskdalemuir and Hartland, we have been able to get a coherent overall picture of the secular variation as measured in Europe. For instance, the first-order time derivative of the Y (east) component essentially displays an increase in two steps from 1900 to 1925 a monotonous decrease from 1925 to 1969 with a regular steplike substructure, and a rapid increase since 1969, followed by a marked reversal of slope in 1979. These results emphasize the internal origin of the 1969 jerk and single out, in Europe, a 1979 event of opposite sign, these two features being quite reminiscent of the behavior of the secular variation during the first quarter of the century.