Commencing with the first self-balancing bridge or potentiometer devised in 1897, a review is given, in historical form, of the several types of servo-operated recording instruments that have been devised since that date. These recorders are all of the type in which a correcting action takes place in the instrument itself, thus making available considerably more power for the recording stylus or pen than is available in direct-acting instruments. A more detailed discussion is then given of the principal types of modern instruments, classified according to the circuit component which is varied to produce balance, namely resistor or potentiometer, capacitor, electromagnetic device. Most instruments record against time as one of the variables, but function plotters, in which the relation between two variables other than time is plotted, are discussed, as also are scanning recorders devised for recording from a large number of input points. Several other special types are also included.
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