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Ohmmeters






Just as ammeters measure current and voltmeters measure voltage, ohmmeters are known to measure resistance. All ohmmeters have an internal source of power (usually a battery) which supplies a known voltage. The meter pointer deflection measures current and the scale is calibrated to read current in relation to voltage or resistance in ohms.

The basic principle of an ohmmeter is: a known voltage, supplied by a battery, is applied to an unknown resistance and the current is measured. It is evident from Ohm's law that when voltage and current are known, we can easily find resistance. As batteries age, their internal resistance increases, and their effective voltage output decreases. To compensate for the aging of batteries, a rheostat is placed in series with the meter. The rheostat is called zero-ohm control. An ohmmeter indicates zero when there is no resistance, but when the meter indicates infinity, there is maximum resistance.

There are three basic types of ohmmeters:

1. Simple series-type ohmmeter;

2. Shunt-type ohmmeter;

3. Voltage-divider ohmmeter.

In a series-type ohmmeter, as resistance increases, current decreases. Another type of ohmmeter is called a shunt-type ohmmeter because the resistor, being measured, is placed in shunt with the meter. The third type of meter is the voltage divider ohmmeter. The unknown resistance and the shunt-resistor form a voltage divider. When different values are inserted for unknown resistance, the voltage across the shunt-resistor will change, and in turn, the current through the meter will change. The series-type ohmmeter is known to be good for high ranges but the voltage divider is considered to be suitable for low ranges.

 

Задание 5

Ответьте письменно на следующие вопросы:

1. What is an ohmmeter used for?

2. What is the source of power in an ohmmeter?

3. What is the basic principle of an ohmmeter?

4. What types of ohmmeters are there?

5. What is a shunt-type ohmmeter?

 

 

Вариант № 5

Задание 1

Перепишите следующие предложения; подчеркните в каждом из них глагол-сказуемое и определите его видовременную форму и залог. Переведите предложения на русский язык. В разделе (б) обратите внимание на перевод пассивных конструкций.

а) 1) The engineers have refused from using these poor quality fuses.

2) Electricity is giving mankind efficient means for meeting the modern society requirements in energy.

b) 1) Today many highly conducting materials are produced on a massive scale.

2) This electric phenomenon was greatly influenced by these sets of external forces.

 

Задание 2

Перепишите следующие предложения; подчеркните Participle I и Participle II и установите функции каждого из них, т. е. укажите, является ли оно определением, обстоятельством или частью глагола-сказуемого. Переведите предложения на русский язык.

1) The chief engineer must give scantlings and drawings if required.

2) The amount of heat generated is very small.

3) The building of the first atomic power station was followed by the research work.

4) The pilot stood on the bridge giving orders to the helmsman.

 

Задание 3

Перепишите следующие предложения; подчеркните в каждом из них модальный глагол или его эквивалент. Переведите предложения на русский язык.

1) Repairs can be executed without special shop equipment.

2) Nowadays power stations should be connected into power grids.

3) The current must be as small as possible not to melt the wires.

4) A fast reactor has to be designed to produce more fuel than it consumes.

5) Considerable development may be done in the energy industry.

 

Задание 4

Перепишите и письменно переведите первые 2 абзаца текста.

 

The Principle of Transformer Action

A transformer is a simple device used in a.c. circuits for the purpose of changing voltage from one value to another. A transformer transfers electric energy from one electric cir­cuit to another without a change in frequency. This energy transfer usually takes place with a voltage change. So trans­formers have to be generally used only with a.c.

A transformer may be defined аs an apparatus without mo­ving parts. It means that mechanical losses are entirely ab­sent. The action of a transformer is based on electromagnetic induction. What does it mean? Each transformer has а core on which the primary and the secondary are wound. The windings are linked magnetically.

The coil on which the a.c. is impressed on the input side of the transformer is called the primary, the one from which the induced e.m.f. is obtained on the output being known as the secondary. The coils may have air cores or they may be wound upon a laminated iron core. In the case of iron core transformers, the secondary voltage is very nearly equal to the primary voltage multiplied by the ratio of the number of the secondary to the number of the primary turns.

There is a limit to the frequencies that can be efficient­ly used in transformers with metallic cores. In case a very high frequency a.c. is used, air core transformers have to be used to eliminate core losses.

An a.voltage being applied to the terminals of the prima­ry, the secondary is open-circuited. An a. magnetic flux is set up around the primary and this induces a back e.m.f. in the primary. A varying magnetic field is established between the two coils. The varying magnetic field will cut across the turns of the secondary and as the result of it an induced e.m.f. is produced across the secondary. This is the principle of a transformer action.

Задание 5

Ответьте письменно на следующие вопросы:

1. What is the function of a transformer?

2. When does the energy transfer take place between two coils?

3. What current is used in a transformer?

4. Why are mechanical losses absent in transformers?

5. What are the main parts of a transformer?

6. What types of transformer cores are there?

 


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