Прочитайте текст и определите какой из заголовков соответствует тексту The polarity of a charge may ...: ответ на тест 781305 - Английский язык

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The polarity of a charge may be determined by means of an electroscope. We charge the electroscope negatively by touching the ball with a rod of hard rubber which is rubbed with flannel or silk. If the unknown charge is brought to the electroscope it will induce on the ball a charge of opposite polarity and on the gold leaves a charge of the same polarity as that of the unknown charge. Therefore, if the unknown charge is negative, the gold leaves will repel each other; if it is positive, they will attract.

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The polarity of a charge may be determined by means of an electroscope. We charge the electroscope negatively by touching the ball with a rod of hard rubber which is rubbed with flannel or silk. If the unknown charge is brought to the electroscope it will induce on the ball a charge of opposite polarity and on the gold leaves a charge of the same polarity as that of the unknown charge. Therefore, if the unknown charge is negative, the gold leaves will repel each other; if it is positive, they will attract.

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Conductors are materials that have a large number of loosely bound valence-ring electrons; these electrons are easily knocked out of their orbit and are then referred to as free electrons. Insulators are materials in which the valence-ring electrons are tightly bound to the nucleus. In between the limits of these two major categories is a third general class of materials called semiconductors.

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The simplest combination of elements constituting an electron tube is the diode. It consists of a cathode, which serves for emitting the electrons, and a plate or anode surrounding the cathode, which acts as a collector of electrons. Both electrodes are enclosed in a highly evacuated envelope of glass or metal. If the cathode is indirectly heated, there must be a heating spiral or a heater. The size of diode tubes varies from tiny metal tubes to large-sized rectifiers. The plate is generally a hollow metallic cylinder made of nickel, molybdenum graphite, tantalum or iron.

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A basic law of electricity states that like charges repel each other and unlike charges attract each other. Electrons emitted from the cathode of an electron tube are negative electric charges. These charges may be either attracted to or repelled from the plate of a diode tube, depending on whether the plate is positively or negatively charged.

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By applying a potential difference (voltage) from a battery or other source between the plate and cathode of a diode, an electric field is established within the tube. The lines of force of this field always extend from the negatively charged element to the positively charged element. Electrons, being negative electric charges, follow the direction of the lines of force in an electric field.