Abstract After some discussion of the general properties of the secular variation field, a graphical method is described of analyzing it in terms of separate dipoles of arbitrary direction. The analysis shows that the major part of the field for epoch 1922.5 is explained by about twelve vertical dipoles below the surface of the core. The relation of this result with other analyses by McNish (1940) and Bullard (1948) is discussed. The depth of the dipoles confirms that the origin of the secular variation must lie in the core of the earth; because of the high electrical conductivity of the core material it must in fact be due to a thin current sheet at the surface of the core, and this interpretation also gives an explanation for the existence of only vertical sources. The presence of only vertical sources, and in particular the presence of several near the equator, does not support the existence of the toroidal field, which is an essential step in Bullard’s (1949) dynamo theory of the main field.