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

CHAPTER II

SUPPLEMENTARY TEXTS

Part I

HISTORY OF ELECTRICITY:

OUTSTANDING SCIENTISTS AND DISCOVERIES

TEXT 1

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.

TEXT 2

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 rate of change of the number of lines of force threading the circuit.

Faraday's Law (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.

TEXT 3

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 30ies 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.

TEXT 4

Kirchhoff's Laws

GUSTAV ROBERT KIRCHHOFF (1824–1887) is a famous German scientist. He graduated from the Kênigsberg 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 formulas /=/j=/2=... and Ea = E1 + E2 + E3 + ... + En are true expressions of Kirchhof's laws as fair as series circuits are concerned.

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.

TEXT 5

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