Chap. 9] SEISMIC METHODS 601 



vanometer equation corresponding to (9-96c?) is 



6 + 26„6 + oilh = ^^ I. (9-96e) 



m 



Eqs. (9-96c, d, and e) may be written in the same general form by 

 denoting the factor of I on the right side as galvanometric magnification 

 factor (static or D,C. sensitivity) Vj, , divided by K: 



b + 2e„6 + 0)^6 = ^" . 7. (9-97a) 



A. 



The solution of this equation for a current of the peak value Im and the 

 frequency w is 



b = Ce-' sin (...( + « + '-^ ^ \ . sin U + ,>,), 



V (,co^ — CO ; + 4€e CO 



(9-976) 



where oidg is the damped frequency of the galvanometer, yj/g the initial 

 phase angle, and (pg the phase shift between galvanometer record and im- 

 pressed current. Galvanometer response curves for steady state conditions 

 (second term in eq. [9-976]) are shown in Fig. 9-107 for various damping 

 rates. It is noted that these curves show the opposite behavior compared 

 with the response of a seismograph. While the dynamic magnification of 

 the latter increases to the static level for tuning factors greater than 1, 

 the dynamic response of the former decreases for tuning factors greater 

 than 1 below the (D.C.) static sensitivity. In a seismograph response 

 curve, the resonance frequencies move toward greater tuning factors with 

 increased damping. In a galvanometer response curve they move toward 

 smaller tuning factors. The resonance frequency of a galvanometer is 



COro = V COo — 2e„ . 



Fig. 9-108 shows response curves of a number of commercial seismo- 

 graph galvanometers. 



4. Over-all response. To obtain the over-all response of a recording 

 channel, that is, the galvanometer deflection for a ground impulse of given 

 amplitude and frequency, the dynamic responses of detector, amplifier, 

 and galvanometer are combined. A similar problem arises in station seis- 

 mology. Various authors have discussed the reaction of a galvanometer 

 coupled directly to a seismograph. Two extreme cases exist: (1) the 

 seismograph mass is large and the galvanometer mass small, so that the 

 seismograph is essentially the driving and the galvanometer the driven 

 unit, with no energy going back into the seismograph from the galvanome- 

 ter; (2) seismograph and galvanometer masses are comparable, so that they 



