The NOAA National Centers for Environmental Information (formerly National Geophysical Data Center) receives magnetograms from over 200 geomagnetic observatories. The Center archives data from 1867 to the present, however the primary holdings are from 1957 to the present. The archives contain over 5,000 reels of 35 mm microfilm copies of magnetograms. Magnetograms are records obtained from an instrument known as a magnetometer which is designed to measure small variations in the components of the geomagnetic field. There are two types of magnetometers, the La Cour and the Ruska. The magnetogram consists of one or more (usually 3) variometers, one for each element usually D, H, Z (less common X, Y, Z) and a recorder. The standard magnetogram usually has all three components recorded simultaneously on one sheet of paper. In addition to the 3 traces, reference lines (baselines) are recorded. The baselines are used in absolute calibration of the magnetogram. The normal recording speed is 20 mm/hour with the drum turning once in 24 hours.