When a permanent record of a magnetic field is required the magnetic flux pattern may be traced out by means of a plotting compass. This consists of a very small magnetic needle pivoted between two glass discs in a brass case (Fig. 31.2). Fig. 31.3
shows how the compass is used. NS is a bar magnet placed on a sheet of white paper. Starting near one pole of the magnet, the position of the ends, n, s of the needle are
marked by pencil dots A, B. The compass is now moved until the end n is exactly over the dot B and the new position of s is marked by a third dot C. This process is continued, and the series of dots obtained are joined, thus giving a magnetic field line which represents the direction of the magnetic flux. Other lines are plotted in the same way, and every line is labelled with an arrow to indicate its direction. Fig. 31.11 and 31.12 are examples of magnetic fields obtained by this method.
If magnetic flux patterns are plotted on a sheet of paper when no magnets are in the vicinity a series of parallel straight lines is obtained directed approximately from S to N geographically. This represents the earth’s magnetic field in a horizontal plane (Fig. 31.4). The earth’s magnetic field is described more fully later in the chapter.
The main advantage of the plotting compass is that it is sensitive and can be used for plotting comparatively weak fields. It is unsuitable for fields in which the direction of the magnetic flux changes rapidly in a confined space, as, for example, in the neighbourhood of two magnets placed close together. Fields such as these are best investigated by the iron-filings method.
