All modern seismographs use semiconductor amplifiers which, like other active (power-dissipating) electronic components, produce continuous electronic noise whose origin is manifold but ultimately related to the quantization of the electric charge. Electromagnetic transducers such as used in geophones also produce thermal electronic noise (resistor noise). The contributions from semiconductor noise and resistor noise are often comparable, and together limit the sensitivity of the system. Another source of continuous noise, the Brownian (thermal) motion of the seismic mass, may be noticeable when the mass is very small (less than a few grams). Presently manufactured seismometers have however sufficient mass to make the Brownian noise negligible against electronic noise, and we will therefore not discuss it here. Seismographs may also suffer from transient disturbances originating in slightly defective semiconductors or in the mechanical parts of the seismometer when these are subject to stresses. The present section is mainly concerned with identifying and measuring instrumental noise.