Description of glass magnetogram scale used by the Department of Terrestrial Magnetism

J. A. Fleming
1919 Journal of Geophysical Research  
To facilitate obtaining the mean ordinate of a 60-minute interval on the magnetograms obtained at the magnetic observatories of the Department of Terrestrial Magnetism, scales have been constructed in the Department's shops somewhat similar to the type introduced by the Potsdam Magnetic Observatory t and as used by the United States Coast and Geodetic Survey. ? The only difference in our glass scale from the Potsdam one arises from the fact that the Potsdam type makes use o[ a double scale, the
more » ... a double scale, the actual mean ordinate in millimeters being obtained from the sum of two readings, whereas we scale the mean ordinate in millimeters direct. The scale is a piece of plate glass !2.5 cm. wide by 20 cm. long, on which are ruled two sets of lines at right angles to each other. The vertical lines parallel to the long sides are 20 mm. apart, corresponding to the distance between hour-breaks on. the Eschenhagen magnetograms, there being however for direct scaling of ordinates an extra vertical line half-way between the vertical lihes inclosing the right-hand 20 mm. of the scale. The horizontal lines parallel to the short sides are I0 mm. apart. The first space at the bottom is subdivided to millimeters by short horizontal lines midway between the vertical lines. (See Fig. !.) A reading is made by laying the scale on the magnetogram with the ruled surface fiext to the paper, the horizontal lines parallel to the base line, the v.ertical lines crossing the base line at the hourmarks, and the space divided to millimeters including the base line. The scale is then moved up or down until one of the horizontal lines is set for the average ordinate for a portion of the curve included between two adjacent vertical lines. With a little practice this can be done readily and with surprising accuracy (except in cases of great disturbance) by making equal the areas, above and below the horizontal line, inclosed by the horizontal line, the
doi:10.1029/te024i004p00154 fatcat:ocovi2ik5nez3osujzlii4xvny