Приложение 4. Тексты для чтения

TEXT N1

Electric Current

This is a circuit. It consists of a voltage source, a resistor and a conductor. A voltage source supplies current. A resistor reduces current. A conductor connects the elements of the circuit.

Compare circuit 'a' with circuit 'b'. What is the difference between them? Current passes through circuit 'a' while no current passes through circuit 'b'. Circuit 'b' has an open. No current through circuit 'b' results from an open. An open and a short are trou­bles in a circuit. A trouble in a circuit may re­sult in no current in it.

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Diodes

We can define electronics as the study of conduction of electricity in a vac­uum, in gases and in semiconductors. The conduction of electricity in a vacuum, for example, occurs in vacuum tubes. Though in some vacuum tubes current flows from one element to the other through a gas

Every vacuum tube diode has a cathode with a heater and a plate. When the circuit is completed (замкнута) the cathode emits electrons. Negative voltage on the cathode repels the electrons. Positive voltage on the plate attracts the electrons. The current flows through the tube. If a negative voltage is applied (приложено) to the plate current does not flow. Thus, a diode permits current to flow in only one direction.

A semiconductor diode also conducts current in one direction, but the physical principles which permit it to do this are different. Diodes are used (используются) as rectifiers of alternating voltages, as detectors of radio signals, as switching devices, etc. (и т.д.)