Measurement Box

a set of specially selected standards of electric parameters that are calibrated to a specific accuracy and are used both individually and in various combinations to reproduce a number of different values of the same parameter. Measurement boxes are used in laboratories and shops in cases where highly accurate changing or adjustment of resistance, capacitance, or inductance in measuring circuits is necessary. The standards are structurally combined in a common housing, which has on its front a switching device or patch board to connect the standards in the required combinations. A distinction is made among switch, jack, plug, and terminal measurement boxes (the last type is rarely used), according to the design of the switching system.

In switch measurement boxes the standards are connected by multiplate phosphor-bronze brushes, which slide over brass eontacts; in jack boxes, tapered brass pins (plugs) are inserted into the sockets of metal plates connected to the standards; in plug boxes, two-pin plugs are inserted into the sockets of a patch board. Most measurement boxes are made with a plug switching device, which is less complicated to manufacture than the lever or jack types and is not inferior in performance.

The standards in measurement boxes are usually made up in groups often (decades) with the same rated value. There may be one to eight decades in a box. In some boxes, to provide smooth regulation of the change in magnitude of the standards, the lowest fixed standard is replaced by a variable standard. Switch boxes have the advantage of rapid switching and convenient readings; their main disadvantage is the substantial contact resistance (about 20 × 10⁻³ ohm). Therefore, they are used mainly where rapid measurements are needed (for example, in the mass inspection and grading of radio components), and jack and plug boxes are used where a minimum contact resistance is of decisive importance (in testing measuring instruments or in highly precise measurements). Measurement boxes having the highest grade of accuracy are usually made with a small number of decades, using plugs; the less accurate boxes are made with many decades and with lever switches. All measurement boxes manufactured in the USSR are standardized and checked periodically.

Measurement boxes are classified as resistance, capacitance, inductance, and mutual inductance (the last is rarely used), depending on the parameter being measured. Resistance boxes consist of a set of coils made of insulated manganin wire connected to the fixed plates of the switching device; resistance boxes are the most numerous group among the measurement boxes. More than 30 types are produced in the USSR. They are divided into seven grades of accuracy: 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1.0. The high-resistance boxes have values from 10 ohms to 1000 gigaohms, and the low-resistance boxes have values from 0.03 to 10.0 ohms. They are made for both DC and AC circuits (in the latter case, from 50 hertz [Hz] to 70 kilohertz [kHz]). Resistance boxes are sometimes used as voltage dividers. In high-accuracy resistance boxes, as well as in AC types, specially wound coils are used to reduce the reactive impedance component, equalizing capacitances are added, and the sections are individually shielded.

Capacitance boxes are sets of electrical capacitors that are used in bridges and equalizing circuits for measurements at frequencies from 40 Hz to 20 kHz. The capacitors in capacitance boxes are very stable and have high insulation resistance. To provide smooth variation of capacitance, a variable capacitor is connected in parallel with the decades. The capacitance boxes made in the USSR have one to five decades; the accuracy grades are 0.05, 0.1, 0.2, 0.5, and 1; and the maximum capacitance ranges from 1 to 111.1 microfarads.

Inductance boxes consisting of sets of inductance coils are used in bridge measurement circuits at frequencies from 20 Hz to 10 kHz. The coils are shielded to avoid the effects of external magnetic fields and also to reduce frequency variation. A variometer is connected in series with the coils of fixed value to provide smooth variation of the inductance. Provision is made in many inductance boxes to replace coils switched out of the circuit with a resistance equal to the winding resistance of the coils, which provides constancy of the ohmic resistance of the measuring circuit. Inductance boxes are produced with five grades of accuracy (0.05, 0.1, 0.2, 0.5, and 1); they have single and multiple decades with maximum values ranging from 11.11 to 111.1 millihenrys.