Chap 9] SEISMIC METHODS 615 



from a mirror attached to the reed to a stationary mirror and thence to 

 the cylindrical lens). Instead of a reed a regular galvanometer may be 

 used to project a time wave on the record. This is the method used for 

 calibrating time lines with a standard fork (see below). Another con- 

 venient way of projecting time lines across a record of almost any width 

 is to mount a neon tube close to the paper and to connect it to an elec- 

 trically driven tuning fork through a high tension step-up transformer. 

 An arrangement now in very common use is a synchronous motor driven 

 by a tuning fork. To its sha^^t is attached a spoked wheel for shadow 

 recording, or a slotted drum for black on white recording. This gives the 

 possibility of making every fifth and tenth time line heavier, which simpli- 

 fies evaluation of the record. Finally, a tuning fork alone may be used 

 to project time lines across the paper by attaching shutters to its tines 

 and projecting a light beam through them onto the cylindrical lens. 



In all timing devices, reeds and forks must be compensated for tempera- 

 ture or be made of metals of low temperature coefficient of elasticity. 

 Timing devices in field recorders should be checked once a month against 

 a standard tuning fork which should be so arranged that it may be readily 

 connected to one of the regular galvanometers in the recording camera. 



G. Calibration of Seismographs 



In station seismology, the calibration of instruments is a comparatively 

 simple matter and involves determinations of natural period, friction, and 

 damping. Natural frequency and friction may be obtained from free 

 vibration records with damping disconnected ; damping is determined from 

 the ratio of consecutive amplitudes with damping mechanism connected. 

 The static magnification V of a seismograph may be obtained by adding 

 a known mass m, and measuring the deflection a of the pen. Then 

 V = amouo/mg, where Wo is the mass and wo the natural frequency of the 

 seismograph. For electrically recording instruments, determination of 

 galvanometer characteristics and transmission constant is necessary in 

 addition to the calibration of the seismometer. Galitzin^^ has described 

 this procedure in detail. 



The calibration of the mechanical prospecting seismograph proceeds in 

 essentially the same manner as calibration of station seismometers. Natu- 

 ral frequency, friction, damping, and magnification are determined as 

 discussed above. Calibration of electrical prospecting seismographs is 

 rendered more elaborate because it has to extend not merely to one, but 

 to as many units as there are recording channels. Further, requirements 



^1 B. Galitzin, VorlesungenueberSeismometrie, Chaps. 6 and7, Teubner, (Leipzig, 

 1914). See also F. W. Sohon, Introduction to Theoretical Seismology, Wiley (1932). 



