Smithsonian Report. 1931. — Heck 



PLATE 2 



1 . Wenner Horizontal Component Seismometer and Equipment 



As in the Galitzln seismometer tlie boom of this instrument carries a coil which moves in a permanent 

 magnetic field as the boom oscillates. The circuit is closed through a sensitive galvanometer carry- 

 ing a mirror for use in photographic registration. Suitable resistances are placed in series with the 

 coils and also in parallel with each branch of the circuit. The latter determine the degree of damp- 

 ing of seismometer or system. In this instrument the coil of the seismometer moves in a radial mag- 

 netic field; that is, the axis of the coil is coincident with the axes of the pole pieces, one pole piece 

 being within the coil and the other without. 



2. Seismographic Recorder, Photographic 



This recording drum carries a sheet of photographic paper upon which a beam of light as reflected from 

 a seismometer or galvanometer is brought to focus. The drum, and with it the paper, is driven on 

 its axis at a speed of 15 to 60 millimeters per minute and at the same time is translated along its 

 axis at a speed of about .'i milUmetprs per minute. The resulting record after photographic devel- 

 opment in the usual manner is called a seismogram. 



