Chap. 9] SEISMIC METHODS 617 



meter grid to give open circuit voltage. With a shaking table, the var- 

 iation of voltage output with ground frequency may be obtained; the 

 amplitude of the table may be measured by various means described 

 in the literature on shaking tables. 



A quantity related to the voltage output is the electrical sensitivity of 

 a detector when it is used as a motor. This is the mechanical deflection 

 for a given current input and is thus proportional to Galitzin's transmission 

 constant, inasmuch as it includes the circuit characteristics and the 

 strength of the magnetic field in the detector. However, this is a test 

 that will give only qualitative comparisons of detectors with similar circuit 

 characteristics. Being a static determination, it is inferior to a shaking- 

 table test. 



2. Amplifiers. In radio practice it is customary to rate amplifiers in 

 terms of decibel gain. A test to determine gain can also be applied to 

 seismograph amplifiers by using a fixed voltage input and determining the 

 output on a vacuum tube voltmeter. Output should be measured in the 

 plate circuit of the last tube, inasmuch as with most galvanometers a 

 stepdown transformer is used. The gain so measured is useful for relative 

 comparisons only; for seismograph amplifiers it is more convenient to 

 measure gain together with galvanometer response as described below. 



3. Galvanometers. Quantities characterizing the action of seismic galva- 

 nometers are natural frequency and damping and static (D.C.) sensitivity. 

 Together they determine the dynamic response. The natural frequency 

 of the galvanometer may be determined from free oscillations by pluck- 

 ing it or giving it an electrical impulse after removing or disconnecting 

 the damping. This test may be made in the regular camera. Possibly 

 simpler is a determination of the resonance frequency by using a beat 

 frequency oscillator. Damping is obtained from overshoot records or from 

 a dynamic response curve. The static sensitivity of a galvanometer is 

 determined by observing the scale deflection for a given current, visually 

 or photographically. If an even rating of galvanometers of different con- 

 struction is desired, their natural frequency, impedance, and optical lever 

 must also be considered. The D.C. test of a galvanometer will generally 

 show whether the unit is performing properly and whether friction is 

 present without requiring a separate friction test. 



4. Combined amplifier and galvanometer response. This test has several 

 advantages over separate amplifier and galvanometer tests: (a) the re- 

 sultant response can be obtained more nearly quantitatively and in terms 

 more closely related to the practical application (galvanometer deflection 

 for a given voltage input of a given frequency) ; (6) tests for quiet operation 



" Heiland, op. cit., 454 (1934), (bibliography on shaking tables). 



