D-c to A-C conversion systems

THE MEASUREMENT of d-c potentials of the order of 10^–6 to 10^–9 volt without the use of sensitive galvanometers has been accomplished only within recent years. Such voltages are encountered in thermopiles, thermocouples, and other similar devices. The small power available from such devices has required the use of delicate galvanometers in conjunction with elaborate mechanical means for recording and controlling functions. Because such galvanometers were of slow time period the recording and controlling action was limited in its speed of response. Direct amplification of the d-c potentials often has been suggested as a means of eliminating the sensitive galvanometer. However, the normal instability and the inherent dependence of d-c amplifiers upon tube characteristics and supply voltage have eliminated them from general use. The ordinary a-c amplifier has so many advantages over the d-c amplifier that it generally is used in all methods of amplifying small d-c potentials. In order that the a-c amplifier may be used, a system for converting the initial d-c potential to alternating current must be accomplished. At this low level of potential a device for changing direct to alternating current is called a “converter.” A device which changes amplified alternating current back to direct current is called a “re-converter.”