The contribution to the measured signal of any desired portion of the ground (as distinct from that of the rest of the ground) in a direct current (dc) well logging system can be computed by using the equivalence between dc and electrostatic fields. Considering a thin cylindrical shell having its axis coincident with that of the sonde, we find that the contribution of the shell usually begins with a small value for small values of shell radii and goes through a few extrema before falling to zero at large radii. In this paper, the radius of investigation of a sonde is defined to be that radius at which the contribution of the cylindrical shell is a maximum and has the same sign as the total measured signal. With this definition, the radii of investigation of the various sondes come out approximately as follows:Tablewhere L is the distance between the extreme active electrodes. Note that, spacing for spacing, the radius of investigation for the unfocused normal sonde seems to be much larger than that for the focused laterolog device.