SEISMIC METHODS 



793 



of the earth. However, it is impossible to construct a pendulum with so large a period 

 that the terms containing (n) are actually negligible. 



Extremely short periods would have the effect of making the terms y and 2nhy 

 unimportant, and Equation 126 may be reduced to an expression similar to 



3' = 



This would mean that the displacement would be proportional to the acceleration of the 

 earth, and this type of seismometer would be called an accelerometcr. In this case (n) 

 becomes large, therefore the displacement would become relatively small, which gener- 

 ally results in a low output of the instrument. 



Moving Conductor Type Seismometer 



The moving coil type of seismometer is probably the most common 

 of the various designs of seismic detecting instruments. Usually this 

 variety of seismometer makes use of the displacement (y) to cut lines of 

 magnetic flux with a conducting coil. If we neglect phase changes, hyster- 



FiG. 479. — Moving coil seismometer. A 

 and A', suspension springs for supporting 

 the mass B and the coil C ; K and K' , flex- 

 ible terminals for coil C ; P and P', pole 

 pieces; Q, centering screw; S, spacer sup- 

 ports. 



esis, eddy current effects, etc., the electromotive force developed will be 

 proportional to the rate at which the lines of force are cut by the coil. 

 This indicates that the electrical output of such instruments may be illus- 

 trated by the equation 



E = -Ry 



where E represents the E.M.F., R the proportional constant, and y the veloc- 

 ity of the inertia mass. Therefore, the electrical output of the moving coil 



