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Electrical Measuring Instruments and Units

Any instrument which measures electrical values is called a meter. An ammeter measures the current in amperes. The abbreviation for the ampere is amp. A voltmeter measures the voltage and the potential difference in volts.

The current in a conductor is determined by two things – the voltage across the conductor and the resistance of the conductor. The unit by which resistance is measured is called the ohm. The resistance in practice is measured with the ohm-meter. A wattmeter measures electrical power in watts. Very delicate ammeters are often used for measuring very small currents. A meter whose scale is calibrated to read a thousandth of an ampere is called a micro ammeter or galvanometer.

Whenever an ammeter or voltmeter is connected to a circuit to measure electric current or potential difference, the ammeter must be connected in series and the voltmeter in parallel. To prevent a change in the electric current when making such an insertion, all ammeters must have a low resistance. Hence, most ammeters have a low resistance wire, called a shunt, connected across the armature coil.

A voltmeter, on the other hand, is connected across that part of the circuit for which a measurement of the potential difference is required. In order that the connection of the voltmeter to the circuit does not change tire electric current in the circuit, the voltmeter must have high resistance. If the armature coil does not have large resistance of its own, additional resistance is added in series.

The heating effect, electrostatic effect, magnetic and electromagnetic effects of electric current are used in order to produce the defleting torque. The resulting measuring instruments are called: (a) hot wire, (b) electrostatic, (c) moving iron, (d) moving coil, and (e) induction. Various types are used with both d.c. and a.c., but the permanent-magnet moving coil instrument are used only with d.c., and the induction type instruments are limited to a.c.

All, except the electrostatic type instruments, are current measuring devices, fundamentally ammeters. Consequently, most voltmeters are ammeters designed also to measure small values of current directly proportional to voltage to be measured.

II. Guess the meaning of international words:

1) instrument; 2) fact; 3) abbreviation; 4) voltmeter; 5) ohm; 6) ohm-meter; 7) wattmeter; 8) galvanometer; 9) shunt.

III. Give the Russian equivalents to the words below:

1) resistance; 2) to offer; 3) scale; 4) to prevent; 5) armature; 6) connection; 7) heating effect.

IV. Give the English equivalents to the words and word-combinations:

1) амперметр; 2) разница потенциалов; 3) определять; 4) чувствительный; 5) градуировать; 6) вставка; 7) катушка; 8) переменный ток (второй термин).

V. Answer the questions:

1. How are electrical values measuring instruments called?

2. How must the ammeter and the voltmeter be connected?

3. What resistance must the ammeter and the voltmeter have?

4. What resulting measuring instruments do you know?

5. What types of instruments are used with both d.c. and a.c.?

6. What instruments are used only with d.c. and limited to a.c.?

VI. Make up sentences corresponding to the contents of the text:

1. A meter 2. An ammeter 3. An ohmmeter 4. A voltmeter 5. A galvanometer measures the resistance very small currents electrical values the current the potential difference in volts
1. The voltage 2. The current 3. The resistance   is measured in ohms in volts in amperes

VII. Describe different types of measuring instruments and units, using the table in Task V.


Part I





GEORGE SYMON OHM (1784–1854) is a famous German physicist. In 1805 he entered the Erlangen University. Though he did not graduate from this University, he managed to write and defend a thesis in 1811. Later, he was a teacher at the gymnasiums of Gottstadt and Wamburg. Beginning from 1833 he became professor at the Polytechnical School in Nuremberg, and since 1849 – at the Mü nchen University.

He is the most famous for establishment of the general law of the electric circuit, stating the relation between resistance, electromotive force, and strength of the current in the electric circuit. The law was discovered experimentally and first formulated in 1826. Further investigations made use of this law. The unit of resistance was named after Ohm at the International Congress of Electricians in1881.


Ohm's Law

One of Ohm's major contributions was the establishment of a definite relationship between voltage, resistance, and current in a closed circuit. A circuit consists of a voltage source and a complete path for current. Ohm stated this relationship as follows:

Current is directly proportional to voltage and inversely proportional to resistance.

As a formula, it appeals like this:

This formula is commonly known as Ohm's Law.

About 1817 Ohm discovered that a simple correlation exists between resistance, current and voltage. That is: the current that flows in the circuit is directly proportional to the voltage and inversely proportional to the resistance. A current is measured in amperes, a voltage, or potential difference is measured in volts. A resistance is measured in ohms.


Faraday's Law

MICHEL FARADAY was a great British physicist, the founder of the theory of electron field, a member of the London Royal Society. He was born in London in the family of a smith. Spending a few years in the primary school, he continued his studies all by himself, reading books and listening public lectures. Greatly impressed by lectures of a well-known English chemist H. Davy, he sent him a letter asking for a job at the Royal Institute. In 1813 Davy gave him a job of a laboratory assistant. Thanks to the brilliant talent of an experimenter, Faraday soon made himself known. All his future scientific work was carried out in the Royal Institute laboratories.

Faraday's law is formulated as follows: (a) the induced E.M.F. in a conductor is proportional to the rate at which the conductor cuts the magnetic lines of force. (b) The induced E.M.F. in a circuit is proportional to the rate of change of the number of lines of force threading the circuit.


EMIL LENZ. Lenz's Law

EMIL LENZ was born on the 12 of February 1804 and died on the 29 of January 1865 in Derpt. He became a prominent Russian physicist, an Academician.

At the age of 16 he entered the Derpt University. In 1823, when being a student, he joined a 3 year round-the-world trip on board of the ship “Enterprise” as a physicist. The chief of the expedition was Kotzebu, a famous Russian seaman and explorer. In 1828 Lenz was elected adjunct-professor of the St.Petersburg Academy of Sciences for his outstanding investigations in geophysics.

In the 30 s of the 19th century, Lenz reorganized a physical laboratory of the Academy of Sciences where he began his famous studies on electricity and magnetism. He discovered the law of the electric current emitting heat in conductors. This law laid the foundation for the discovery of the Law of conservation and conversion of energy.

The direction of the induced current is such that its effect opposes the change producing it. The right-hand rule enables one to predict the direction of the induced current, and may be shown to conform with Lenz's law.

The induction coil, the dynamo, the transformer, and the telephone are practical application of electromagnetic induction.


Kirchhoff's Laws

GUSTAV ROBERT KIRCHHOFF (1824–1887) is a famous German scientist. He graduated from the Kö nigsberg University in 1846. Since 1850 he had been an extraordinary professor of physics at the University of Breslau, and since 1854 – an ordinary professor of experimental and theoretical physics in Heidelberg University, in 1875 he became the chief of the Chair of mathematical physics in Berlin University.

His first works (1845–49) were dedicated to studies of the electric current in various kinds of conductors, series and parallel circuits, and to distribution of electricity in the conductors. Together with Bunsen, he was the author of spectral analysis.

G. R. Kirchhoff expanded and clarified Ohm's law with two statements which may be paraphrased as follows:

1. The current entering a point is equivalent to the current leaving the point.

2. The sum of the voltage drops around a closed loop is equal to the applied voltage.

Kirchhoff intended his statements to apply to all circuits.

The two main principles of circuit analysis are:

(1) Kirchhoff's Current Law. The sum of the currents directed away from the junction is equal to the sum of the currents directed toward the junction.

(2) Kirchhoff's E. M. F. Law. The sum of the voltage drops around any closed loop of a network equals the sum of the voltage rises around this loop.




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