2. The circuit consisting of the (ac power line) , together with the power transformer of a measurement system instrument, provides another path through which conductively coupled interference can be introduced into the measurement system. The sources of this interference include current spikes and fluctuations in the power line voltage which occur when motors, temperature-controlled furnaces, and so on, are operated or turned on and off. Such interference is transmitted by the power line throughout the common power distribution circuit. If a measurement setup is connected to this same power distribution circuit, these noise spikes will thereby be conductively coupled to the (ac power) input of the measuring instruments operating from the power line. A large capacitance (typically 1000pF) may exist between the primary and secondary winding of the power transformers of electronic instruments(i.e., oscilloscopes, amplifiers, etc.), and thus the current spikes can be strongly coupled to the internal power conductors of the instrument.Further coupling of this interference to the wire carrying the measured (input) signal can occur through capacitive or inductive effects, or through the degradation of the power supply output also caused by this interference. One technique used to suppress this type of interference is to build the instrument power transformer with capacitive interference shielding (called Faraday shielding) between their primary and secondary winding. This shielding consists of conducting (metal) foil and permits magnetic energy to pass, but attenuates capacitive interference. The conducting foil is connected to the power-line ground. Thus the electrostatically induced currents are carried to ground through the foil rather than through the circuitry of the transformer secondary winding capacitance of the transformer (i.e., from 1000 pF to less than 1.0 pF). The problem of the conductively coupled noise spikes is thereby effectively reduced. A second (and quote obvious) technique for reducing power transformer conductively coupled interference is to remove known sources of such interference from the local (ac power distribution network.