SEISMIC METHODS 



797 



magnet, which can be easily adjusted until the desired damping and fre- 

 quency are obtained. 



Theory of Reluctance Seismometers 



The reluctance-type seismometers are those wherein the reluctance of the mag- 

 netic path is caused to vary by the relative movement of the inertia reactor with 

 respect to the seismometer housing. Numerous instruments of this type are used for 

 both earthquake and exploration seismology. 



In one type of reluctance seismometer, two permanent magnets are supported by 

 two springs which are attached to the case. (Figure 488.) The laminated armature 

 is mounted in the air gap between the magnets and is rigidly connected to the case. 

 The motion of the earth's surface during the passage of seismic waves produces a 

 relative motion of the magnets with respect to the armature which causes an induced 

 E.M.F. in the armature coil. 



Fig. 487. — Magnetic circuit of an electromag- 

 netic duplex reluctance seismometer. (After 

 Scherbatskoy and Neufeld, Geophysics.) 



C 



Fig. 488. — Reluctance seismometer, duplex- 

 type. A, laminated armature; B, armature coil; 

 C, aluminum case, waterproof type; M, per- 

 manent magnets; Si and S^, suspension springs 

 for the magnetic system (end view of springs) ; 

 T, brass yoke. 



To determine the quantities T, V, D for a reluctance type seismometer, it is 

 convenient to refer to the equivalent magnetic circuit shown in Figure 487. The 

 symbols denote the following quantities :t 



F = magnetomotive force of each magnet. 

 R = reluctance of each magnet. 



Ra 



Re 



reluctance of the armature. 

 = reluctance of each air gap when .y =r 0, where the reference line 

 .y = is located midway between the magnets when they are unde- 

 flected. From symmetry Rg also equals the reluctance of the two 

 air gaps in series above, or below, the neutral line s =^ 0. 

 I = one-half the vertical distance between the pole pieces of the two 

 magnets when the magnets are undeflected. 



^' =z reluctance of two air gaps in series above the armature correspond- 

 ing to a displacement j of the reference line. 



/ 



t Scherbatskoy and Neufeld, "Fundamental Relations in Seismometry," Geophysics, Vol. II, 

 No. 3, July, 1937, pp. 192-198. 



