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UNIT 19
I. Read the text
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 re-
sistance 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 se-
ries 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 ef-
fects 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 de-
vices, fundamentally ammeters. Consequently, most voltmeters are ammeters
designed also to measure small values of current directly proportional to volt-
age 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) connec-
tion; 7) heating effect.
IV. Give the English equivalents to the words and word-combinations:
1) амперметр; 2) разница потенциалов; 3) определят; 4) чувствитель-
ный; 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 the resistance
2. An ammeter very small currents
3. An ohmmeter measures electrical values
4. A voltmeter the current
5. A galvanometer the potential difference in volts
1. The voltage in ohms
2. The current is measured in volts
3. The resistance in amperes
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 re-
lationship 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:
Voltage (in volts)
Current (in amperes)
Resistance (in ohms)
This formula is commonly known as Ohm's Law.
About 1817 Ohm discovered that a simple correlation exists between re-
sistance, current and voltage. That is: the current that flows in the circuit is di-
rectly 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
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 conduc-
tor 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 in-
duced 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 Janu-
ary 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 stu-
dent, 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 con-
ductors. This law laid the foundation for the discovery of the Law of conserva-
tion 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 sci-
entist. He graduated from the Kкnigsberg 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 mathe-
matical 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 ap-
plied 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
A GREAT INVENTION OF A RUSSIAN SCIENTIST
Radio occupies one of the leading places among the greatest achieve-
ments of modern engineering. It was invented by Professor A. S. Popov, a tal-
ented Russian scientist, who demonstrated the first radio – receiving set in the
world on May 7, 1895.
And it is on this day that the anniversary of the birth of the radio is
marked.
By his invention Popov made a priceless contribution to the development
of world science.
A. S. Popov was born in the Urals, on March 16, 1859. For some years he
had been studying at the seminary in Perm, and then went to the University of
St. Petersburg. In his student days he worked as a mechanic at one of the first
electric power – plants in St. Petersburg which was producing electric lights
for Nevsky Prospekt.
After graduating from the University in 1882, A. S. Popov remained there
as a post – graduate at the Physics Department. A year later he became a lec-
turer in Physics and Electrical Engineering in Kronstadt. By this time he had
already gained recognition among specialists as an authority in this field.
After Hertz had published his experiments proving the existence of elec-
tromagnetic waves, A. S. Popov thought of the possibility of using Hertz
waves for transmitting signals over a distance. Thus the first wireless (radio)
receiving set was created. Then Popov developed his device and on March 24,
1896 he demonstrated the transmission and reception of a radiogram consisting
of two words: Heinrich Hertz. On that day the radio-telegraphy was converted
from an abstract theoretical problem into a real fact.
A. S. Popov did not live to see the great progress of his invention. In the
first decrees the Soviet Government planned the development of an industry
for producing radio equipment, the construction of radio stations. All this was
put into practice on a scale which had greatly surpassed plans for the radiofica-
tion of the country.
Popov’s invention laid the foundation for further inventions and im-
provements in the field of radio engineering. Since that time scientists all over
the world have been developing the modern systems of radiotelegraphy,
broadcasting, television, radiolocation, radio-navigation and other branches of
radio-electronics.
TEXT 6
CHARLES COULOMB
CHARLES COULOMB (1736–1806), a member of the Paris Academy of
Sciences, an outstanding French physicist in the period from 1785 to 1789
stated the law of electrostatic and magnetic interaction. His work in this field
laid foundation for the future theoretic investigations in the electrostatics and
magnetstatics.
Coulomb’s law is one of the principal laws of electrostatics. It estab-
lished a relationship between the force of interaction of two static electric
charges, their quantities, and the distance between them. According to Cou-
lomb’s law the absolute value of the force of repulsion of two like charges or
the force of attraction between two unlike charges e l and e 2, which size is
much less than the distance between them, is inversely proportional to the
square of the distance between them. He also stated the laws of rotation, dry
friction, laws of interaction between magnetic poles. All these laws were named
in honor of Ch. Coulomb.
TEXT 7
ANDRE MARIE AMPERE
ANDRE MARIE AMPERE (1775–1836) was an outstanding physicist and
mathematician of French origin. He is one of the founders of modern electrody-
namics. He was born in aristocratic family in Lyon. By the age of 14 he has read
all the 20 volumes of «The Encyclopedia» by Diderot and D’Alambert. His sci-
entific interests were very diverse.
In 1801 Ampere headed the Chair of Physics in Burge, in 1805 he became
a teacher of physics at the Polytechnical School in Paris. Since 1814 he was
elected Member of The Institute, which later transformed into the French
Academy of Sciences. After 1824 he occupied the post of professor at the Ecole
Normale in Paris.
Ampere’s studies on the effects of the electric current flow on the mag-
netic needle were his greatest contribution to physics. In 1820 in the report to
the Paris Academy, he made the announcement of the so-called “Ampere
Rule”, which is since used to define the deflection of the needle affected by the
electric current. This led him to the discovery of interactions between electric
currents. The fundamental laws of this interaction got his name.
TEXT 8
GEORGE SYMON OHM
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 Nьrenberg, and since 1849
– at the Mьnchen University.
He is most famous for establishment of the general law of the electric cir-
cuit, 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 Electri-
cians in 1881.
TEXT 9
JAMES CLERC MAXWELL
JAMES CLERC MAXWELL, a British physicist, was born in 1831. In
1847–1850, he studied at the Edinborough University and later in Cambridge.
On graduating from the Cambridge University, he was offered a post of a
teacher there. In 1860 he headed the Chair of Physics in the King’s College in
London. In 1871 he went back to Cambridge where he headed a newly-
organized laboratory named in honor of H. Cavendish.
His scientific interests lay in the field of electro-magnetism, molecular
physics, optics, mechanics and other. Maxwell published his first scientific pa-
per when he was only 15. He founded the theory of electro-magnetic field,
the electromagnetic theory of light. He is credited with the studies of the
Saturnus rings. He described all known facts of electrodynamics by means of
system of equations, known as Maxwell’s equations of electrodynamics.
TEXT 10
WORLD BRIGHTEST ELECTRIC LAMPS
The world’s brightest lamp, able to light an area of 250 acres was pro-
duced by the Moscow Electric Lamp Works not long ago. It was designed by
Victor Vasiliyev.
The lamp, which is named after the bright star Sirius is a three – phase
200 – kilowatt discharge lamp. The working part of the lamp is a double
walled quartz tube which is 10 inches in diameter and about 40 inches long.
The lamp is started by a special high voltage flash and cooled by water circu-
lating between the inner and outer tubes.
One of these lamps is now installed nearly 200 feet above ground level in
the engineering pavilion of the Industrial Exhibition Moscow. The Sirius lamp
can be particularly useful on big construction sites.
TEXT 11
EARLY HISTORY OF ELECTRICITY
History shows us that at least 2,500 years ago the Greeks were already fa-
miliar with the strange force (as it seemed to them) which is known today as
electricity. Generally speaking, three phenomena made up all of man's knowl-
edge of electrical effects. The first phenomenon was the familiar lightning