The literature on earth noise over the spectrum of periods from 0.1 s to 12 h is reviewed and presented as accelerations which, when expressed as spectral‐noise density, allow all seismographs to be considered on a common basis of sensitivity. Their ultimate sensitivity is limited by thermal agitation of their suspended masses and is calculable by considering the energy flow to and from their lossy elements in accordance with principles elucidated by Nyquist. Practical instruments, including those with feedback loops, are considered as examples. Seismometers and accelerometers are treated here as resonant systems having one degree of freedom. The electrical outputs of their transducers are classified as ‘velocity’ or ‘displacement.’ The dynamic range required to record these outputs depends on the range of accelerations imposed on the instrument and upon the combined filtering effects of the resonant system and transducer. Gravitational variation, tilt, and tidal accelerations are considered as they affect the instrumenta