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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 4 (1937), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The classical theory of electromagnetic seismographs is discussed with special reference to the properties of combinations of seismometer and galvanometer in which the damping of either or both components in the system is very much greater than critical. The paper includes a graphical method of determining approximate response curves for such seismographs.The treatment of passive systems is extended to cover cases in which the galvanometer light spot is made to energize a photoelectric amplifier, and in which the amplifier output is fed back into the mechanical system. It is shown that feedback into the galvanometer changes the apparent period or damping constant, but that such changes can only lead to types of seismograph which could, in principle, be constructed without the use of amplifiers. Feedback into the seismometer does, however, produce new effects. In particular it is shown that feedback proportional to the velocity of the galvanometer spot is equivalent to a change in the galvanometer reaction, such that the reaction constant λ may be given any desired positive or negative value. Large positive values of λ, in conjunction with large damping coefficients, lead to extremely wide-band characteristics. The argument is illustrated by experimental data on a seismograph constructed in Ottawa which yields an almost flat response to ground velocity from 0.5s to 800s period, and it is concluded that this pass-band could, if necessary, be extended by at least a decade at each end.
    Type of Medium: Electronic Resource
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