SEISMIC METHODS 



801 



dt 



(170) 



Equations 169 and 170 are the fundamental equations of motion of the mechanical 

 and electrical parts of the seismometer. Equation 169, which describes the motion of 

 the mechanical part, contains one term, — Kmi, which depends on the electrical circuit, 

 and Equation 170, which describes the conditions in the electrical circuit, contains one 

 term, KmS which depends on the relative velocity of the moving element in the me- 

 chanical system. 



It is possible to solve Equations 169 and 170 so as to obtain the current in the elec- 

 trical circuit as a function of the relative velocity of the moving element in the me- 

 chanical circuit, provided an explicit form is assumed for the mechanical force /. 

 This is equivalent to determining the current in the electrical circuit as a function 

 of the velocity of the ground at the seismometer station.* 



Fig. 489. — Piezoelectric type seismometer, a, waterproof case; b, clamp screw for locking 

 instrument; c and d, double connector for filament and plate supply and pre-amplifier output; 

 e, amplifier tube; f, grid bias battery; g, transformer; /;, piezoelectric crystal; i, carrying rope. 



Piezoelectric Seismometer 



The piezoelectric seismometerf consists essentially of a crystal of aniso- 

 tropic material such as quartz, tourmaline, Rochelle salt, etc., and an inertia 

 reactor. The motion of the earth produces a relative acceleration of the 

 inertia reactor with respect to the crystal. This relative acceleration in turn 

 produces a potential difference between opposite faces of the crystal. 



Several advantages are claimed for the piezoelectric pickup; chief of 

 which are : ( 1 ) the seismometer does not have moving parts such as 

 elements supported by elastic springs; (2) the voltages generated are 



* The procedure for carrying out the simultaneous solution of Equations 169 and 

 170 is indicated by Scherbatskoy and Neufeld, loc. eit. 



t W. P. Welch, "Accelerometer (Piezoelectric)," U. S. Patent 3,411,401, Nov. 19, 1946. 



