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3. In what way are the elements connected?
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3. In what way are the elements connected?
4. What do the readings on the scale show?
4. Read the text and find in it the answers to the questions that follow it.
Rheostat
A rheostat is a resistor whose resistance value may be varied. Thus, a rheostat is a variable resistor.
It is used to change the resistance of circuits, and in this way to vary the value of current.
A rheostat consists of a coil and a switch. Take into consideration that wire used for the coil must have a very high resistance. When a rheostat is used its terminals are connected in series with the load. The switch is used to change the length of the wire through which the measured current passes. The resistance may be changed to any value from zero to maximum.
The longer the rheostat wire used in the circuit, the greater is the resistance.
1. What type of resistor is a rheostat?
2. What is a rheostat used for?
3. In what way does a rheostat vary the value of current?
4. What elements does a rheostat consist of?
5. In what way are the terminals connected with the load?
6. What is the function of the switch?
5. Read the text and find in it the answer to the question that follows it.
Voltage Values
Voltages up to about 250 V are called low. The common electric lighting circuit operates either at about 127 or 200 V, and the voltage used on the main circuit of large houses is usually the same. One can get an electric shock, when one touches an uninsulated wire of such a circuit.
Voltages above 250 V are high voltages. They are used in industry. Medium-powered motors are usually operated at 380 V. Large motors are supplied by voltages of from about 500 up to 6, 000 V.
What have you read about?
6. Read the text and find in it the answer to the question that follows it.
High-Frequency Current
Alternating current with frequency of 50 c/s is widely used in industry. Therefore this frequency is called an industrial frequency and' the current, an industrial frequency current.
During sound transmission, current flowing in telephone wires changes with the frequency of sound oscillations, which ranges from 50 to 10, 000 c/s. The currents of such frequencies are called audio- or low-frequency currents.
Radio transmission is based on the use of alternating currents with frequencies of hundreds, thousands, millions and even tens of million cycles per second. These currents called high-frequency currents are produced by means of an oscillatory circuit consisting of a coil and a capacitor. Moving along the turns now in one direction now in another, the electric charges oscillate in an oscillatory circuit.
By what means are high-frequency currents obtained?
7. Read the text and find in it the answers to the questions that follow it.
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HV Power Transmission
A high-capacity hydrogenerator produces an a. c. current at 22, 000 V. The current with the potential difference of 220, 000 V is produced by means of the transformers at a step-up station and then transmitted over the power lines.
The current potential difference is lowered to medium 6, 600 V at the main step-down substation at the end of the line. From here the power is transmitted to the next substations. Transformers stepping the voltage down from 6, 600 V are installed at those substations.
Due to voltage conversion, alternating current is used widely in industry. Direct current for battery charging for trams, trolleybuses and electric locomotives is changed from alternating current by means of rectifiers.
1. Where is the current potential difference lowered?
2. Where is the main step-down substation installed?
8. Read the text and find in it the answers to the questions that follow it.
Meters
One of the important things that an engineer should take into consideration is «how much»? How much current is this circuit carrying? What is the value of voltage in the circuit? What is the value of resistance? In fact, to measure the current and the voltage is not difficult at all. One should connect an ammeter or a voltmeter to the circuit and read off the amperes and the volts.
Common ammeters for d. c. measurements are the ammeters of the magneto-electric system. In an ammeter of this type an armature coil rotates between the poles of a permanent magnet; but the coil turns only through a small angle. The greater the current in the coil, the greater the force, and, therefore, the greater the angle of rotation of the armature. The deflection is measured by means of a pointer connected to the armature and the scale of the meter reads directly in amperes.
When the currents to be measured are very small, one should use a galvanometer. Some galvanometers detect and measure currents as small as 10-11 of an ampere per 1 mm of the scale.
A voltmeter is a device to be used for measuring the potential difference between any two points in a circuit. A voltmeter has armatures that move when an electric current is sent through their coils. The deflection, like that of an ammeter, is proportional to the current flowing through the armature coil.
A voltmeter must have a very high resistance since it passes only very small currents which will not disturb the rest of the circuit. An ammeter, on the other hand, must have a low resistance, since all the current must pass through it. In actual use the ammeter is placed in series with the circuit, while the voltmeter is placed in parallel with that part of the circuit where the voltage is to be measured.
In addition to instruments for measuring current and voltage, there are also devices for measuring electric power and energy.
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