8. Reed tbe text and find in it tbe answers to the questions tbat fotlow iL

Meters

One of the important things that an engineer should take into con- sideration is “how much?” How much current is this circuit carrying? What is the value of voltage in the circuit? What is the value of resis- tance? 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 ammetem for d. G. lTleasurements are the ammetcrs of _the magnate-electric system. In an ammeter of this type an armature coil rotates between the poles of a permanent magnet; but the coil tums _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 ar- mature 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

small as 10 " ' of an ampere per 1 mm of the scale.

measure currents as

A voltmeter is a device to be used for measuring the potential differ- ence between any two points in a circuit. A voltmeter has armatures that move when an electric current is sent through their coils. The de- flection, 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 se- ries with the circuit, while the voltmeter is placed in pamllel 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.

1. WJtat is the above article about?

2. What is a voltmeter used for?

9. Read the text and find in it the answcm to the questions that follow iL

Negative Trensconductence Oscillator

A negative transconductance oscillator consists of a pentode with the screen and suppressor grids coupled together. The screen is more positive than the plate. The cathode supplies electrons to both the screen and the plate. If the screen voltage rises, this increase is transferred over to the suppressor. It becomes more positive than before. A large quantity of electrons passes to the plate and a small quantity of electrons passes to the screen. Thus, the screen current is reduced. A negative resistance characteristic is produced in the screen circuit at the terminals.

A tuned circuit is connected to the terminals and in this way oscilla- tions are produced.

1. In what way is the screen current reduced?

2. What docs negative transconductcncc oscillator consist of?

-)19-

10• Reed tbc text aod fiod ia it tbc aa9wcr to tbc question tbat follows ih

Wbat are tbe Names of Oscillators?

Many types of oscillator circuits are used nowadays. Their tube types, tank cimuits, and feedbacks are different.

Examine the diagrams given below. What are the names of the os-

cillators? To answer this question mad the following texts.

1. The Electron-coupled Oscillator uses a multi-grid vacuum tube with a cathode and two grids operating in common. In this device the plate cimuit load is coupled to the oscillator through the electron stream. The diagram shows a tetrode variant using a series-fed oscilla-

tor.

2. The Dynetron Oscillator uses the negative resistance chamc- teristic of a tetrode tube to eliminate the resistance of its tank circuit in order to keep oscillations.

3. The Blocking Oscillator uses a plate transformer to provide the feedback voltage.

4. A Multivibmtor is a form of oscillator using two RC-controlled

$. The Tuned Plete - Tuned Grid oscillator has parallel or reso- nant cimuits in both plate and grid circuits. The necessary feedback is supplied by the plate-to-grid interelectrode capacitance.

Nsmc the five oscillators in Fig. 20.

JJ. Reed the text end find in it the answer to the question that Follows It.

Soler Energy

The Sun radiates to our planet great amounts of energy. This energy input fuels all our biological processes and is the original source for hydropower, wind power, and fossil fuel resources. Environmentally and economically, the Sun is our greatest energy soume. Why then is the direct use of solar energy so limited for industrial, domestic, com- mercial, and transportation purposes?

First, solar energy arrives in small quantities — only about one kW per square meter; second, it does not arrive at all dunng night hours. Third, it is very difficult to tmnsform it into useful energy forms except low-grade heat.

W’hy is the direct use of solar energy limited nowadays?

12. Read the text and find in it the answer to the question that Follows it.

In the electric generating field, as well as in the electric-transmission and stomge-technology areas, considerable attention is focused on the use of superconductivity conductors. Cooling an electric conductor to temperatures close to absolute zero (—273 C) results in the conductors’ losing their resistance to electric current. Accordingly, their ohmic losses also drop greatly. Current densities of tens of millions amperes per sq cm can be tolerated in such conductors.

Cryogenic technology being now mastered, one can foresee the pos- sibility of supemooling not only generators but also underground transmission lines and storage coils.

Arc these statements true or false?