Практическое
занятие № 6/145.
Тема: Классификация
электроизмерительных приборов. Говорение.
.Цель:
совершенствование лексических навыков.
Перечень
оборудования для проведения работы: тетрадь,
письменные принадлежности, словарь.
Задание:
выполнить упражнения
1. Прочтите
и переведите текст.
ELECTRICAL MEASURING INSTRUMENTS
General Electrical Principles.
To measure the magnitude of any phenomenon calls for the use of the
effects produced by it. For the measurement of a mass, for example, we determine
its weight; or we might utilize the force required to give it a
certain acceleration. Again, for the measurement of temperature
we use the expansion of solids, liquids, or gases, the change in electrical
resistance of a wire, or the thermo-electromotive force produced at the
junction of two conductors, etc. For the everyday measurement such indicating
instruments are most convenient, in which the amount of the quantity to
be measured is directly shown by the position of a pointer on a
graduated scale or dial.
For the production of such instruments,
some effect is employed which enables the phenomenon to be measured to produce
a mechanical force tending to move the pointer along its scale; this is
resisted by a controlling force which tends to move the index in the opposite
direction, towards some zero position. The actual displacement of the
index, or deflection, is the resultant of these two forces, and is greater the
greater the magnitude of the deflecting force or of the phenomenon to
be measured.
As a matter of fact, one single principle
is known to underlie all electromagnetic instruments, viz. that the
current-carrying circuit tends to enclose as large a magnetic field as possible.
In the moving needle galvanometers the magnetic needle turns so that
more of its lines or force pass through the coil, while in the moving coil
instrument the coil sets itself so as to enclose as much of the field of the
magnet as possible. In the soft iron instruments the iron moves so as to
increase the magnetic flux produced by the coil, and in the dynamometer the
moving coil turns so that its magnetic effect increases that of the fixed coil.
Should the conducting circuit be made entirely of flexible material in a
uniform field, it would become circular in order to enclose the maximum
possible area, while, if the current flowed in a liquid conductor –
such, for instance, as mercury – it would actually try to reduce its section
so as to shorten the path of the lines of force around it. To such an extent is
this the case that is difficult to pass a large current along such a conductor
owing to the tendency of the mercury to contract and break the circuit.
2. Прочтите и выучите:
measure
– измерять
determine
– определять, устанавливать
utilize
– использовать
thermo-electromotive force
– термоэлектрическая сила
pointer – стрелка
dial – шкала
displacement – смещение
viz. – а именно
magnetic flux
– магнитный поток
uniform field
– однородное поле
mercury
– ртуть
3.
Прочтите текст и, основываясь на его содержании, заполните таблицу.
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construction
|
operation principle
|
usage
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ammeters
|
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voltmeters
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