Measure of electrical resistance

Measures of electrical resistance are called exemplary resistors if they are designed, manufactured and passed state verification for this purpose. ^[1] Divided into 2 groups:

• unambiguous electrical resistance measures (OMES) - resistance coils;
• multi-valued electrical resistance measures (MMES) - resistance stores.

The range of resistance values ​​reproduced by:

• resistance coils - from 10 ⁻⁴ to 10 ¹² Ohms
• resistance shops - for one stage of a decade from 10 ⁻⁴ to 10 ¹² Ohms ^[2]

The history of resistance measures in the 19th century is closely connected with the development of resistance measurement units. In 1843, Charles Wheatstone proposed a device for measuring resistance - a measuring bridge , also known as the Wheatstone bridge. To quickly and conveniently balance the bridge, Wheatstone designed 3 types of rheostats , one of which resembled resistance shops, which in 1860 was improved and widely distributed by Werner von Siemens . ^[3] According to some sources, Wheatstone did not use variable resistors to balance the bridge, and the first adjustable shoulder bridge was designed by Werner Siemens in 1847. ^[4]

In 1848, Boris Jacobi proposed his own unit for measuring resistance and made standards of copper wire of uniform cross section 25 feet long. (7.61975 m) and weighing 345 grains (22.4932 g). The standards were coils with wire, filled with an insulating composition and placed in wooden boxes. Jacobi sent copies of his standard to I.H. Poggendorf , who made a number of copies of the Jacobi standard and sent them to the most prominent physicists in Europe.

In 1857, Jacobi demonstrated a device containing 3 groups of 11 identical coils of silver wire with a thickness of 0.07 inches in a common wooden case. The coils of the first group carried 4 inches of wire, the second - 40 and the third - 400. ^{[5] [6]}

Prior to the beginning of the 1900s, nickel silver was mainly used for the manufacture of resistance coils due to its high resistivity . The stability of resistance over time and the TCS left much to be desired and with the advent of alternatives, the use of the alloy was abandoned. In the last decade of the 19th century, platinum was widely used along with nickel silver - a similar alloy composition supplemented with tungsten (Cu - 60%, Ni - 14%, Zn - 24%, W 1 - 2%), more stable and with less TCR. From the mid-1890s, constantan and manganin began to be used for the manufacture of coils. The latter satisfied most of the requirements for materials for the manufacture of model resistances: high resistivity, low TCR, low thermoEMF paired with copper. Sensitivity to air humidity and atmospheric corrosive agents was compensated by shellac insulation of the wire. ^[7]

Resistance measures are usually made of manganin since the latter:

• has a large electrical resistivity of 0.43-0.48 · 10 ⁻⁶ Ohm · m;
• a very small temperature coefficient - of the order of 1 · 10 ⁻⁵ K ⁻¹ ;
• low thermoEMF paired with copper.

Moreover, depending on the resistance, they usually use ^[1] :

• from 10 ⁻⁴ to 10 ⁻² Ohm - sheet manganin for resistances;
• from 10 ⁻² to 10 ² Ohms — manganin wire wound bifilarly;
• from 10 ³ to 10 ⁵ Ohms - manganin wire wound according to Chaperon;
• over 10 ⁷ Ohm - manganin microwire in glass insulation. ^[eight]

Highly stable measures can also be made of nichrome .

Unambiguous measures

A winding of manganin wire is wound on a metal or porcelain frame, the ends of which are soldered to the clamps. The coil frame is attached to the housing with holes for better cooling of the winding. Measures with a nominal resistance of 10 ⁵ ohms or more have an electrostatic screen, the role of which is usually played by the device case. In some designs, the housing is filled with transformer oil , kerosene or silicone oils , which increases the moisture resistance of the insulation and improves the heat transfer conditions of the winding. Early measures used to measure high currents (up to 1000 A) could have devices for mixing the filling fluid and a coil for the forced circulation of cooling water ^[9] . To control the temperature of the coil, a socket for installing a thermometer is provided. The coils are equipped with four clamps, two of which are called current and are designed to include a model coil in the current circuit, the other two are called potential. Potential clamps are designed to measure the voltage drop across the coil resistance. Exemplary manganin resistors can be loaded up to 1 W in air, and up to 4 W in an oil bath. ^[one]

Ambiguous measures

Classification

By the number of decades:

• one-decade;
• multi-decade.

By design of the switching system:

• plug - measures are connected using conical brass rods (plugs) that are inserted into the nests of metal plates connected to the measures. Contact resistance with a good surface condition and sufficient pressure is approximately 1x10 ⁻⁴ Ohms; with surface contamination and slight pressure, the contact resistance can increase hundreds of times, as a result of this, significant errors can occur. The oldest type of switching system.
• Fork - measures are connected using a two-pin plug inserted into the slots of a typesetting board;
• lever - measures are connected using multi-plate brushes made of phosphor bronze, sliding on brass contacts. Due to the fact that the width of the brushes exceeds the gap between the contacts, this is the only type of switching system that provides switching without breaking the circuit.
• electronic - switching is carried out using low-temperature precision relays controlled by a microcontroller. Such a store requires a power source (mains or battery) for its operation.

In the USSR in the Russian Federation, by the end of the 20th century, plug and fork stores were completely replaced by lever shops, but such devices are present in the assortment of foreign manufacturers.

Currently, resistance measures are applied to:

• verification and calibration of ohmmeters and megohmmeters ;
• verification and calibration of secondary instruments designed to work with resistance thermometers .

In accordance with the current GOST 23737-79 “Measures of electrical resistance. General specifications "the following characteristics are standardized:

1. for all kinds of resistance measures
   1. permissible deviation of the actual resistance value from the nominal (accuracy class);
   2. permissible change in resistance in percent for a year (instability);
   3. the limit of permissible additional error caused by a change in the temperature of the ambient air (environment) (in the range between the upper point of the range of normal conditions of use and a point within the range of operating conditions of use corresponding to the maximum resistance);
   4. the limiting value of the dissipation power and the permissible time of its exposure (irreversible changes in resistance after such an exposure should not exceed the established limit, expressed in fractions of the main error)
   5. the limit of permissible additional error when changing the power dissipation (ranging from nominal to any value not exceeding the maximum power);
   6. the value of thermal contact EMF for multi-valued measures with a resistance of the senior decade stage of 10 ⁴ Ohms or less and unambiguous measures with a nominal resistance of less than 10 ⁴ Ohms;
2. for resistance measures intended for use on alternating current
   1. time constant limits
   2. upper limit of the frequency range in kHz
   3. the initial inductance and inductance when the resistance is turned on is not more than 1 Ohm (including the initial inductance) for multi-valued measures with a decade resistance of 100 Ohms or less.
   4. the limit of the permissible additional error caused by the change in frequency (from zero to the upper limit of the frequency range)
3. only for lever MMES with the resistance of the senior decade of 10 ⁴ Ohms or less
   1. the average value of the initial resistance (that is, the resistance when all ten-day switches are set to zero);
   2. variation of initial resistance caused by a change in the transient resistances of the contacts of switching devices

For quick and reliable inclusion of unambiguous model resistances into the circuit, the conclusions of the latter were made of thick copper bars, the ends of which bent down (as shown in the figure) and were placed in cups with mercury . ^[10] The ends of the conductors, to which it was required to attach a model coil, were lowered into the same cups. Subsequently, this method was abandoned due to the high toxicity of mercury .

1. ↑ ^{1 2 3} Biryukov S.V., Cheredov A.I. Metrology: Texts of lectures. - Omsk: Publishing House of OmSTU, 2000, - 110 s (neopr.) . Humer library.
2. ↑ GOST 23737-79 "Measures of electrical resistance. General specifications "
3. ↑ Mario Llozzi History of Physics - M.: Mir, 1970 - p. 261
4. ↑ TD Lockwood Electrical Measurement and the Galvanometer: Its Construction and Uses. second edition - New York, 1890 - p . 56 (neopr.) . Internet Archive: digital library of free Books, Movies, Music & Wayback Machine. Date of treatment May 3, 2013.
5. ↑ material about B. S. Jacobi on a site dedicated to the history of electrical engineering (neopr.) (Inaccessible link) . "E-Scientist.RU". Date of treatment May 3, 2013. Archived on September 10, 2011.
6. ↑ this device should not be called a “resistance store” because it did not have special switching elements
7. ↑ Bob Mills. The RESISTANCE BOX . Australasian Telephone Collectors Society Inc. (1996). Date of treatment January 12, 2014.
8. ↑ Kushnir V.F. Electroradio-measurements - L.: Energoatomizdat - 1983 - p. 15
9. ↑ Voinarovsky P.D. ,. Electrical measuring devices // Brockhaus and Efron Encyclopedic Dictionary : 86 tons (82 tons and 4 additional). - SPb. , 1890-1907.
10. ↑ Resistance Store // Brockhaus and Efron Encyclopedic Dictionary : 86 tons (82 tons and 4 additional). - SPb. , 1890-1907.

Literature

• Shop of resistance // Brockhaus and Efron Encyclopedic Dictionary : 86 tons (82 tons and 4 additional). - SPb. , 1890-1907.
• Voinarovsky P. D .,. Electrical measuring devices // Brockhaus and Efron Encyclopedic Dictionary : 86 tons (82 tons and 4 additional). - SPb. , 1890-1907.
• Measuring shop // Big Soviet Encyclopedia : [30 t.] / Ch. ed. A.M. Prokhorov . - 3rd ed. - M .: Soviet Encyclopedia, 1969-1978.
• Electrical measurements. Means and methods of measurement (general course). Ed. Shramkova E. G. Textbook for technical colleges - M.: Higher School, 1972-520, C
• Biryukov S.V., Cheredov A.I. Metrology: Texts of lectures. - Omsk: Publishing House of OmSTU, 2000, - 110 s (neopr.) . Humer library.

Regulatory and technical documentation

• GOST 23737-79 "Measures of electrical resistance. General specifications "
• GOST 8.237-2003 “State system for ensuring the uniformity of measurements. Measures of electrical resistance are unambiguous. Verification Method
• MI 1695-87 "Measures of electrical resistance are multi-valued, used in DC circuits. Verification Method. ”

• Electrical resistance
• Ohmmeter
• Megohmmeter
• Measuring bridge

• TD Lockwood Electrical Measurement and the Galvanometer: Its Construction and Uses. second edition - New York, 1890 - P.50-64 (English) . Internet Archive: digital library of free Books, Movies, Music & Wayback Machine. Date of treatment May 3, 2013.
• Bob Mills. The RESISTANCE BOX . Australasian Telephone Collectors Society Inc. (1996). Date of treatment January 12, 2014.