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EXPLORATION GEOPHYSICS 



all others : Recording apparatus for seismic field work should be : 

 (1) rugged, (2) compact, (3) completely light-proof, (4) unaffected by 

 extraneous vibrations and field transportation, (5) of sufficient recording 

 paper capacity, (6) designed to use spools or rolls of recording paper 

 which are commercially available, and (7) preferably equipped for visual 

 observation of the recording traces while recording. 



Galvanometers 



The galvanometers are usually of the Einthoven type or of 



the 



D'Arsonval type. From a mechanical viewpoint, the Einthoven or string 

 galvanometer is simpler than the D'Arsonval or moving coil galvanometer, 

 but generally not as satisfactory for field work. 



Einthoven or String Type. — This type of galvanometer consists 

 of a single permanent or electro-magnet with a number of wires strung 



Fig. 500. — Stringing of an Einthoven galvanometer element 

 eter "strings"; b, ivory spacer posts; c, strings of fine copper 

 proper tension. 



(7, fixcJ terminals inv galvanom- 

 wire; d, adjustable terminals for 



through an air gap in the magnet. Each wire carries one of the amplified 

 seismometer outputs to be recorded. A beam of parallel light rays passing 

 through the air gap causes the strings to cast shadows on the photographic 

 paper. These shadows reproduce as white traces when the record is 

 developed. The strings may be oil damped. 



