TEXT 13. AMMETERS AND VOLTMETERS
The action of almost all the types of measuring instruments is such that an electric current as distinct from an electric potential, is primarily responsible for the ultimate mechanical force required to produce movements of the instrument pointer. This has an exceedingly important influence on the practical forms of ammeters and voltmeters. By this is meant, that all, except the electrostatic type of instrument, are fundamentally current-measuring devices. They are fundamentally ammeters. Consequently, most voltmeters are merely ammeters so designed as to measure small values of current directly proportional to the voltages to be measured. It is only natural, therefore, that voltmeters and ammeters should be classed together.
Ammeters,which are connected in series in the circuit carrying the current to be measured, are of low electrical resistance, this being essential in order that they cause only a small drop of voltage in the circuit being tested, and accordingly absorb a minimum power from it.
Voltmetersare connected across, that is, in parallel with the circuit points where the voltage is to be measured, and are of high resistance, in this case sufficiently high so that the current flowing in the voltmeter, and the power absorbed from the circuit, are as small as possible.
The principle upon which both of these devices operate is essentially the same as that of the electric motor, differing from the mo-
tor, however, in the delicateness of their construction and the restrained motion of the rotating armature.
A coil of fine copper wire is so mounted between the two poles of a permanent magnet that its rotation is restrained by a hairspring. The farther the coil is turned from its equilibrium or zero position, the greater is the restoring force. To this coil is fastened a long pointer at the end of which is a fixed scale reading amperes if it is an ammeter or volts, if it is a voltmeter. Upon increasing the current through the moving coil of an ammeter or voltmeter the resultant magnetic field between coil and magnet is distorted more and more. The resulting increase in force therefore turns the coil through a greater and greater angle, reaching a point where it is just balanced by the restoring force of the hairspring.
Whenever an ammeter or voltmeter is connected to a circuit to measure electric current or potential difference, the ammeter must be connected in series and the voltmeter in parallel. As illustrated in Fig. 4 the ammeter is so connected that all of the electric current passes through it. To prevent a change in the electric current when making such an insertion, all ammeters must have a low resistance.
| Pole Pieces |
Ammeter
Voltmeter
Fig. 4
Hence, most ammeters have a low resistance wire, called a shunt, connected across the armature coil.
A voltmeter, on the other hand, is connected across that part of the circuit for which a measurement of the potential difference is required. The potential difference between the ends of the resistance R, being wanted, the voltmeter is connected as shown. Should the potential difference across R2 be desired, the voltmeter connections would be made at C and D, whereas if the potential difference maintained by the battery were desired, they would be made at A and D. In order that the connection of a voltmeter to a circuit does not change the electric current in the circuit, the voltmeter must have a high resistance. If the armature coil does not have a large resistance of its own, additional resistance is added in series.
Very delicate ammeters are often used for measuring very small currents too. A meter whose scale is calibrated to read thousandths ofan ampere is called a milliammeter, one whose scale is calibrated in millionths of an ampere being known as a microammeter or galvanometer.
Notes
ultimate['altimit] - основний; кінцевий
exceedingly[ik'skdirnli] - надзвичайно
consequently['konsikwantli] - отже
resistance[ri'zistans] - опір
to absorb[ab'so:b] - поглинання
sufficiently -достатньо
hairspring -волосяна пружина
equilibrium - рівновага
to distort[dis'to:t] - перекручувати
to restore[ris'ta:] - відновлювати
insertion -прокладка
hence[hens] - з цього часу
to shunt -заміняти
ampere['ampea] - ампер
electric current - електричний струм
Comprehension questions:
1. What is the peculiarity of the action of almost all the types of measuring instruments?
2. What is the difference in a circuit connection between ammeters and voltmeters?
3. What is the principle upon which both devices operate? Give their characteristics.
TEXT 14. TRANSFORMERS
Unlike the generator, a transformer cannot be used to convert mechanical energy into electrical energy, it being able to transform electrical energy from one circuit at the same or some other voltage.
Essentially, a transformer consists of two coils, not electrically connected to each other, but wound over a common core. The core may also be of open type, or it may be merely a tube of some insulating material, the latter being referred to as an air core. If a varying voltage be applied to
| - |
the primary coil, the electromagnetic field set up around the coil will rise and fall in accordance with the e. m. f. variations applied. This moving field cuts the turns in the secondary coil and induces an e. m. f. therein. The value of this induced e. m. f. depends upon the strength of the applied e. m. f. and the ratio of secondary turns to primary turns. Should there be twice as many turns in the secondary as in the primary, the voltage in the secondary would be twice that applied to the primary. If there were half as many turns in the secondary, the voltage would be half that applied to the primary. This voltage step up or step down in proportion to turn ratio will hold good for all combinations. Where the voltage is raised, however, amperage is lost in the same proportions, and vice versa. Therefore, the power in watts supplied to the transformer is the same as that drawn from it, assuming the transformer to be 100 percent efficient. The copper losses, or ohmic resistance of the windings, and the core losses due to the induction of eddy currents in the core material, as well as hysteresis or molecular friction caused by changing polarity of the current applied, all combine to reduce modern transformer efficiency to about 90 percent.
Transformers are classed according to the use they are designed for. Where it is desired to step up a low-voltage a. c. supply to a value useful for radio receivers and transmitters, a power transformer is used. The windings are sufficiently heavy to carry the current without undue heating, and the secondary may consist of two or more separate windings to provide various voltages from one input source.
As the iron core increases, the inductive reactance of the transformer increases, and inasmuch as this type of reactance also increases with the frequency of the applied alternating current, there is a limit to the frequencies that can be efficiently used in transformers with metallic cores.
Notes
transformer[traens'foima] - трансформатор
to convert -перетворювати
circuitt'sa:kit] - ланцюг
essentially- по суті
core [ko:] - стержень
tube [tju:b] - електронна лампа
E. m. f., emf
(electromotive force) - електрорушійний
to induce[jn'dju:s] - спонукати
therein[dear'in] - тут, там
ratio- пропорція
amperage -сила струму
vice versa -навпаки
eddy currents -вируючі струми
undue -невідповідний;надмірний
input - подача
inasmuch -оскільки
reactance -реактивний опір
frequency -частота
Comprehension questions:
1. What is the difference between generators and transformers?
2. What does transformer consist of?
3. What does occur if a varying voltage be applied to the primary coil?
4. In what way are the transformers classed?