The Type 1605-A Impedance Comparator (Figure 1) is designed to measure and indicate on meters the magnitude and phase-angle differences between two external impedances. No bridge-balancing operation is necessary, and these measurements may therefore be made rapidly and easily. The instrument is basically a special, self-contained, bridge measurement system, consisting of a signal source, a bridge, and a detecting circuit. The bridge proper consists of the two external impedances to be compatfed and two highly precise unity ratio arms. Since these ratio arms are equal to within one partin 106, the accuracy of impedance measurement depends, largely on the precision of the external standard. The detector sensitivity permits measurements to 0.01% and 0.0001 radian, an order of magnitude more accurate than that of most precision impedance bridges. In general, this bridge circuit is not adjusted for a balance, but instead the unbalance voltage is measured to give the required impedance difference information. The detector is phase sensitive, and selects those vector components of the unbalance voltage that are proportional to theimpedance- magnitude difference in percent and the phase-angle difference. The combination of four decade frequencies from 100 cps to 100 kc, with a very wide impedance range and several difference ranges, results in an instrument of wide versatility and flexibility. 1.2 PURPOSE. The combination of speed, wide range and high accuracy in the Impedance Comparator bring precision to rapid production testing and speed to delicate laboratory measurements. Obvious uses are the rapid sorting, matching, and selecting of components. Inspection of the most precise components can be made rapidly over wide ranges of impedance and frequency. Components of poor phase angle (lossy capacitors or inductors, or resistors with a reactive component), which could cause circuit difficulties as easily as could components of improper value, can be rejected without the need for specialized test procedures. The precision possible when precise impedance standards are used results in a system that can replace many intricate measurement setups in the laboratory and simplify the measurement procedure. Because the impedance-difference information is provided continuously, the measurement of changes in impedance due to environmental changes is greatly simplified. With a suitable recorder, a record of the data may be easily made. With an adjustable standard, the instrument may be brought to a null, in which case transfer impedances of three terminal networks may be determined. When the instrument is nulled, impedance shunting of the detector does not affect the K^1coj »!*ci thv sffcct of iRipvdmiCs shiiutiug jjig ratio s»*ins is usually negligible due to the tight coupling of these arms. Manyother special measurements are possible, including checking the tracking of ganged components, adjusting balanced transformer windings, and loss measurements on dielectric materials. The process of comparison is basic to impedance measurement, and a precise comparator should find many other interesting uses.