An electronic galvanometer is a vacuum tube or transistor amplifier, followed by a milli ammeter may be employed as a detector. Some devices have arbitrary scale values, sometimes with variable gain and are designed simply as balance detectors. Sensitivity of such devices may be of the order of a microvolt per scale division or better, and the input Â impedance is in the range of megaohms, so the current drawn from the measured source is usually negligible. Such instruments have the advantage of ruggedness,portability that permit their use under conditions that would be extremely difficult for a suspended galvanometer. Another possibility is a VTVM, if sufficiently low scales are provided. Instruments are available with ranges susch as 5-0-5 or 10-0-10 microvolts giving a fraction of microvolts per division, the input impedance is in Mega ohms.
On any such electronic galavanometers with high input impedance, the indication is practically the Thevenin voltage produced by the unbalance of the bridge or potentiometer, unless the circuit itself is of very high impedance. Electronic detectors are often useful; however there may be difficulties connected with their use. It is desirable to ground the circuit , if possible at one detector terminals, otherwise stray capacitance might introduce errors in measurement.