The equations for dynamical equilibrium of a geophone are derived by means of the Lagrange method and applied to the particular cases of a moving armature geophone, moving conductor geophone, and electrostatic geophone. The galvanometer for seismic recording is usually of the moving coil type and its equations are similar to those of a moving conductor geophone. The magnetic type geophone as well as the galvanometer consist essentially of two dynamical systems characterized by an asymmetric coupling: “the transducing resistance.” A brief analysis is given of the problem of determining the response of a seismograph to a given earth motion.