714 



DARBYSHTRE 



[CHAP. 20 



recorded. The conventional type seismographs used hitherto gave very little 

 or no electrical output biit this is not the case with modern instruments. The 

 strain seismograph used by Gutenberg and designed by Benioff (1955), has 

 already been mentioned. Very sharply tuned seismographs have also been 

 used by Donn, Ewing and Press (1954) to help to locate different microseism 

 sources. In Great Britain, a new type of seismograph has been developed by 

 Tucker (1958). This has a high magnification (up to 18,000) and a flat response 

 from 1 to 10 sec period. It consists of a pendulum of 1.6 sec natural period, and 

 the displacement of the bob relative to its mean position is measured by a 

 transducer which gives a 4000 c/s signal balanced to zero output in the mean 

 position. This signal is amplified and rectified taking account of phase, giving 



Fig. 9. Photograph of correlation meter. (After Iyer and Hinde, 1959.) 



a voltage proportional to the bob displacement. This voltage is fed back to a 

 coil attached to the bob and with one of its sides in the field of a permanent 

 magnet, the sense is such that the force on the coil reduces the defiection. By 

 arranging suitable circuits in the feed-back part, the period and characteristics 

 of the pendulum system can be altered at will. Both E-W and N-S components 

 are incorporated in the same model. A vertical-component seismograph works 

 on a similar principle, the pendulum arrangement is supported in a horizontal 

 position by a "zero length" spring. 



Instruments have also been devised to analyse the microseisms quickly. 

 Iyer and Hinde (1959) have published an account of a simple instrument for 

 doing this. It consists essentially of a moving coil movement with a needle 



