SEISMIC METHODS 



817 



Another galvanometer used extensively in seismic recording is illus- 

 trated in F"igure 502. A set of 25 hermetically-sealed plug-in galvanometers 

 constructed of stainless steel are confined to a width of approximately ZVz 

 inches in this design. These galvanometers are electromagnetically damped 

 and may be constructed with natural frequencies ranging from 15 to 1500 

 cycles per second. 



Recording Cameras for Visual Analysis of Records.—One of the 



several types of individual recording cameras available for seismic work 

 is shown by Figure 503. The front panel of the camera is composed of two 

 hinged doors. The viewing screen is permanently secured to the center 

 of the panel. The right-hand door allows ready access to the galvanometers, 

 while the left-hand door services the supply and re-roll magazines for 

 photographic paper. 



Fig. 503. — Interior view of recording oscillograph. (A) photographic paper maga- 

 zine; (B) galvanometer compartment containing multi-element unit; (C) ground glass 

 viewing screen; (D) galvanometer element consisting of coil and suspension; (E) drawer 

 compartment containing synchronous timing system; (F) paper cutting and perforating 

 mechanism. (Courtesy of William Miller Corporation.) 



The light source is housed in a separate compartment. A beam of light 

 passes through a diaphragm, impinges upon the focusing mirrors of the 

 multiple galvanometer bank, and is reflected from these mirrors in indi- 

 vidual ribbons of light, with their long dimension vertical so as to come to 

 a focus in sharp vertical image lines at the distance of the recording paper. 

 In front of the recording paper is placed a cylindrical lens having a length 

 equal to the width of the recording paper, and with its axis horizontal. 

 This lens focuses the individual galvanometer traces (vertical lines) to 

 small bright spots on the recording paper. The lower portions of the ribbons 

 of light reflected from the galvanometer mirrors are intercepted by a 

 narrow reflecting mirror and are projected on the ground glass viewing 

 screen at the front of the camera, where they may be observed while a record 

 is being taken. 



A temperature-compensated 100 c.p.s. tuning fork is mounted in a 

 thermally insulated compartment and is equipped with either a carbon 

 button or a vacuum tube drive. The output of the fork is amplified and 



