I. Прочитайте и запомните следующие слова и словосочетания

electromotive force	электродвижущая сила, ЭДС
to denote	1) указывать на (что-л), показывать; 2) означать, обозначать, значить
amount	1)количество; 2) сумма, итог; 3) значительность, важность
voltaic cell	гальванический элемент, химический источник тока
thermoelectric device	термоэлектрическое устройство, прибор
to gain	1) получать, приобретать; 2) добывать; 3) наращивать, добывать; 4) выигрывать, добиваться
misnomer	неправильное употребление имени или термина
dimension	1) размерность; 2) размер, габарит; 3) измерение
to confine	1) ограничивать; 2) держать взаперти
armature	1) арматура; 2) эл. якорь ; 3) эл.броня
polarity	1) полярность; 2) поляритет, полярное преобразование
magnetic flux	магнитный поток
coil	1) спираль; 2) виток; 3) рулон; 4) бухта проволоки, шнура; 5) катушка
to reduce	1) понижать; 2) ослаблять; 3) уменьшать, сокращать
to spin	1) крутить(ся), вертеть(ся); 2) выдавливать на станке; 3) вращаться ; 4) свинчивать или развинчивать трубы

II. Прочитайте и переведите текст Electromotive Force

This term is used to denote the force which moves or tends to move electricity from one point in a conductor to another. The analogy of the water pipes will be useful in explaining the nature of electromotive force, or E.M.F., as it is usually abbreviated. The rate of flow of water in pipes depends not only upon the pressure or head of water, but upon the resistance offered by the pipes. Thus if the pipes are choked with dirt, they will not deliver the same amount of water as when the pipes are clear, although the head of water is the same in each case. So with electricity in a conductor, the quantity that passes any point depends upon the electromotive force and the resistance of the conductor. Just as water flows from a cistern at a high level to a tap at a low level, so electricity may be regarded as flowing from the positive pole of a battery or dynamo towards the negative pole, the flow being maintained by the difference of pressure, or the difference of potential as it is called, between the two poles.

Electromotive force (emf, ) is a term used to characterize electrical devices, such as voltaic cells,thermoelectric devices, electrical generators and transformers, and even resistors. For a given device, if an electric charge passes through that device, and gains an energy, the net emf for that device is the energy gained per unit charge. This has units of volts, or joules per coulomb, and hence can be thought of as a voltage induced by the device in question. Since force has the unit of the newton, emf is a misnomer, but one that over time has resisted change.

In most circuits current is driven by a so-called "source of emf", which usually is a voltaic cell (or battery, which consists of voltaic cells in series and/or in parallel) or the power company. For a voltaic cell the source of emf is the chemical reactions that occur at each of the electrode-electrolyte interfaces, so that a voltaic cell can be thought of as two "surface pumps" of atomic dimension. The reactions at the electrode-electrolyte interfaces provide the "seat" of emf for the voltaic cell. For the power company, the source of emf is electromagnetic induction, which is more extended than an atomic size, but nevertheless is confined to the power generation building, usually many miles from the user.

The counter-electromotive force (abbreviated counter emf, or CEMF ) is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is represented by Lenz's Law of electromagnetism. Back electromotive force is a voltage that occurs in electric motors where there is relative motion between the armature of the motor and the external magnetic field. One practical application is to use this phenomenon to indirectly measure motor speed and position. Counter emf is a voltage developed in an inductor network by a pulsating current or an alternating current. The voltage's polarity is at every moment the reverse of the input voltage.

In a generator using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil; the motor is acting like a generator.. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force". (by Lenz's law.) With a lower overall voltage across the armature, the current flowing into the motor coils is reduced.

If it is assumed that a motor is 100% efficient with no friction or windage losses, the speed of the armature will increase until the back electromotive force is equal to the applied electromotive force, i.e. there will be no net electromotive force, no current flow and hence, no net force. The armature will spin at a constant rate, of its own accord.