A device that approximates and whose output is proportional to an integral of the input signal. A low pass filter. In a superheterodyne radio it amplifies a fixed frequency lower than the received radio frequency and higher than the audio frequency. Intermittent problems are often difficult to locate because of the random nature. They often don't occur when the technician is present.
Every source has some resistance in series with the output current.
When current is drawn from the source some power is lost due to the voltage drop across the internal resistance. Usually called output impedance or output resistance. A semiconductor material with electrical properties essentially characteristic of ideal pure crystal. Essentially silicon or germanium crystal with no measurable impurities. A signal applied at the inverting input will be given 180 phase shift between input and outptu.