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The development of research in the field of electrical and magnetic phenomena and enhance their practical applications have necessitated the development of methods for measuring basic electrical quantities, and the creation of special electrical devices.

The principle of operation of the first electric appliances was based on the deviation of the magnetic needle by an electric current. However, such devices are essentially the only indicators of current.

The first indicator of electrical current multiplier was I. X. Shveyggera created in 1820 m. It is a framework that consists of a few turns of wire, which was placed inside a magnetic needle. Experiments have shown that an increase in the number of turns of the coil amplifies the effect of the current on the arrow. However, due to the influence of terrestrial magnetism on the magnetic needle of the multiplier his testimony was inaccurate.

In 1821 was found (Ampere), the possibility of eliminating the influence of terrestrial magnetism using an astatic pair consisting of two magnetic needles, fortified on a common axis and parallel to one another, and the arrow poles facing in opposite directions.

In 1825, Italian physicist L. Nobili astatic couple has combined with the multiplier and made more sensitive device. For practical measurements were necessary instruments with direct-reading, pre-calibrate, for which it would be possible to count the measured values.

It is clear that the greatest need first arose in the direct measurement of the value of the current flowing through the conductor. The first step in this direction was the creation of switch devices in which the sine or tangent of the deflection angle of the arrow was proportional to the current. Such devices are called the sine galvanometer and the tangent-galvanometer.

In 30-ies of XIX century. widely spread-tangent galvanometer Helsingfors University professor I. Nervandera. However, direct determination of the measured value with the help of such a device has been difficult. It was necessary to calibrate the device.

The first attempt to calibrate the galvanometer was made in 1839 BS Jacobi. By including the galvanometer in series in a single chain with voltameter, Jacobi was able to establish a relationship between the amount of current is determined by means of electrolysis, and the deviation of the magnetic needle of the galvanometer. Thus, it was established the concept of graded by the amount of current the galvanometer and introduced the method of grading, widely spread.

Of great importance for the development of electrical instrument was the creation of an electromagnet (1825, William Sturgeon). Since the second quarter of XIX century., Electromagnets have been widely used in various electrical appliances and devices.

The presence at the disposal of physics and electrical engineering devices and components such as solenoid, relay, astatic couple SLR range, springs to create a counter-torque, allowed to go to the construction of more sophisticated electrical devices. Already in the first half of XIX century. are more sensitive and accurate galvanometer elektrodinamometr static galvanometer and so on. n.

Advances in the field of theoretical and experimental studies led to the discovery of new methods for electrical and magnetic measurements. These include the development of ballistic measurement method (E. X. Lenz, 1832), the compensation method (I. Poggendorff, 1841), a bridge measuring circuit (Charles Wheatstone, 1843), and others.

Ballistic method based on measuring in a ballistic galvanometer number of pulses of electricity induced by a change in flux linkage of the coil windings included in the chain of the galvanometer.

Ballistic galvanometer different from the usual mirror galvanometer much greater moment of inertia of the moving part of the device that allows the movement of its frame after the cessation of current in it. Fundamentals of ballistic galvanometer theory developed by Lenz.

The action of the induced current was considered as an instant hit (hence the name of the device: the ancient Greek ballista-throwing weapon), under whose influence multiplier arrow deflected at a certain rate, and this rate is proportional to sin a / 2, where a - angle of greatest deflection of the arrow.

Compensation measurement method greatly improves the measurement accuracy. The main advantage of this method is that the measurement is made at the full compensation of the currents in the individual branches, when the light through the current does not pass (zero display). This excludes the error caused by the voltage drop in the wires, the measurement of the inevitable ordinary voltmeter.

Compensation method is widely used in low-power circuits, where the switch is connected with the measuring devices power consumption commensurate with the power consumed in the circuit being measured.

The development of telegraphy and several other electrical applications demanded a device for measuring the resistance. In the 40-60-ies developed the first design resistors ( "voltagometr" BS Jacobi), rheochords (I. Poggendorff), Shops of resistance, and other similar devices.

During the reporting period stabilized names of basic electrical quantities, gradually set the terms: electrical resistance, electromotive force, current, amount of electricity, etc .; the use of these terms in the scientific and technical literature is gradually acquiring a definite character.

During the 1830-1860 biennium. many instruments have been built and developed methods for measuring basic electrical quantities. Electrometry solid foundation was laid, but it is still mainly concentrated in the laboratories.

Only since the 70s of the last century is very fast implementation of measurement methods in wide practice. With regard to the units for measuring electrical quantities, during the period under discussion in this area there was unsteady position and many researchers have been entered into a scientific turn their units.

The concept of electrical units while sometimes identified with the standard concept. For example, to measure the electrical resistance of E. Lentz X. (1838) used unit sales as a benchmark, which had the character of a calibrated length of copper wire 1 foot taken from the assortment of the time.

In 1848, Jacobi established the normal standard of resistance, which has been extended in many Russian and European laboratories and had a look and feel that matches our modern ideas of the benchmark.

"Unit Jacobi resistance" was produced in the form of a coil of copper wire 25 feet long, weighing 22.5 grams and a diameter of 0.67 mm and placed in a special box and filled with an insulating compound. The standards of this unit is manufactured serially.

Along with identity Jacobi also existed the resistance unit of Siemens , Wheatstone and others. The confusion in this area looked like at Jacobi expression, the situation in public life, which has developed would be if everyone enjoyed their special meters or pounds.

The need to establish common international units grew more and more; in particular, due to the fact that the Telegraph has turned into an international means of communication, there is a need to unify the resistance units.

A big step forward in the establishment of generally accepted units was made in 1861 by the British Association for the Advancement of Science, establish a special committee to develop the question of the standard unit of electrical resistance. The Committee included such prominent scholars as William Thomson (Lord Kelvin), J. Maxwell et al.

The Committee has expanded its program of work and is not limited to the problem of unity and resistance standard, introduced in 1870 by the Association of the project, in which it recommended a system of electrical units, based on the absolute electromagnetic system.

The Committee suggested to the following practical units: Resistance - "OMAD" or "om" , e. d. s. - "V" , capacitance - "farad" . Units power and quantity of electricity was derived from the previous ones, and for their special names were suggested. With regard to the real standard unit of resistance, the Committee stopped on the reference of the wire (alloy of silver and 2 parts 1 part of platinum).

The proposal of the British Association Committee may be only advisory in nature, though mainly for England. The problem acquired great spear, and became absolutely necessary and inevitable consideration of its international scale.

At a 1881 International Congress of electricians discussion of units it became the focus of attention of scientists. Special Commission, established by Congress recognized that electrical units must be based on the metric system, and approved the proposed Committee of the British Association for the basic units: centimeter, gram, second.

Of the two systems - the absolute electrostatic and electromagnetic absolute - absolute recommended for use as an electromagnetic system more convenient for measurement purposes; it does not exclude the possibility of use and absolute electrostatic system.

The Commission has also resolved the issue of the current units and the quantity of electricity, and gave them the name "amps" and "pendant" . The final resolution of the issue of units and the reference received at the Chicago Electrotechnical Congress in 1893

Source: LD Belkind, A. Veselovsky, IY Confederate, YA Shneiberg. History of energy technology . M., L .: Gosenergoizdat, 1960.