804 



EXPLORATION GEOPHYSICS 



fication. The carbon seismometer, however, is seldom used for deep work 

 due to the high noise level caused by "frying" of the grains. These micro- 

 phonic noises may be minimized to a certain extent by using two carbon 

 buttons in a push-pull circuit in the primary of the transformer. 



Photoelectric Seismometer 



The photoelectric seismometer is a displacement type instrument where- 

 in the relative motion of the inertia mass with respect to the seismometer 



case modulates the light which falls 

 on a photoelectric cell. The instru- 

 ment is shown schematically in Figure 

 492. The operation of the seismom- 

 eter may be summarized as follows : 

 The motion of the earth's surface pro- 

 duces a relative displacement of a 

 mass, thereby actuating a pivoted mir- 

 ror through a connecting link. Light 

 rays from a small lamp are focused on 

 the mirror by a condensing lens and 

 are then reflected to a photoelectric 

 cell. The output of the photoelectric 

 cell depends on the displacement of 

 the mass actuating the pivoted mirror.* 



A seismometer of this type has 

 certain definite advantages : namely 

 constancy of response characteristics, 

 negligible temperature coefficient, and 

 a relatively high output level. One 

 disadvantage of the instrument is the 

 necessity for a local battery (usually 



placed adjacent the seismometer) or an extra pair of conductors to light 



the filament. 



9///////////////////////////////////m//J 



Fig. 492. — Schematic diagram of a photo- 

 electric seismometer. A, case; B, light source; 

 C, condensing lens; D, mirror; E, flexible 

 support; F, photoelectric cell; G, impedance 

 matching transformer between cell and line; 

 H, stationary mass; I, connecting link be- 

 tween mass and mirror; J, diaphragm support 

 for mass. 



Electronic Seismometer 



A seismometer with high output utilizes the mechano-electronic trans- 

 ducer tube,** illustrated in Figure 493. This electronic triode tube is provided 

 with a plate shaft which extends through the center of a thin metal dia- 

 phragm. Displacement of the plate shaft changes the distance between the 

 fixed grid and the plate, and thereby changes the plate current. The tube 

 has a deflection sensitivity of 40 volts per degree deflection of the plate 

 shaft, which has a minimum free cantilever resonance of 12,000 cps. By 



* The cell may be connected directly to a transformer for proper impedance match. 

 ** Radio Corporation of America mechano-electric transducer, tube No. 5734. 



