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Chapter LXVI
A D-C generator is an electric machine which converts the mechanical energy of its prime mover into electric energy which is delivered to the consumer. For its operation, a D-C generator depends on electromagnetic induction. It consists essentially of a rotating armature carrying a winding, and a stationary field structure with electromagnets which produce the magnetic field. The armature core is a cylinder assembled of separate 0.5.-mm-thick silicon-steel punchings, which are insulated from one another by a coating of varnish or thin paper. When the core is assembled, the notches on the punchings form slots which receive the armature winding. The armature shaft carries a commutator consisting of separate cooper segments each soldered to a definite portion of the armature winding. The poles of the electromagnets are of steel and are bolted to the yoke of the machine. The yoke is usually cast of steel. In small machines, the yoke and poles are cast integral. In the larger machines, the poles are assembled of silicon-steel punchings. The poles carry field coils wound with insulated copper wire. The current traversing the field coils sets up a magnetic field around the poles. The armature circuit is connected to the load circuit by means of brushes. The brushes are carried in brush-holders which are mounted on brush-holder studs or brackets. When the armature rotates, its winding cuts across the magnetic field and an alternating current is produced in it. So that the terminal voltage can always act upon the external load in the same direction, some device must be inserted between the armature winding and the terminals. This device must reverse the connection of the winding conductors to the external circuit at the instant when the voltage of the conductors is zero and changing in direction. Such a device is called a commutator. There are some types of direct-current generators. The MMF necessary to establish the flux in the magnetic circuit of a DC generator can be obtained by means of (1) a permanent magnet; (2) field coils excited by the generator itself; (3) field coils excited by some external source. A permanent-magnet generator consists of an armature and one or several permanent magnets encircling the armature. In the case of separately excited generators, the field coils may be excited by any D-C constant -potential source as a storage battery, a D-C generator (called an exciter), a rectifier, etc. Where the field coils are excited by the generator itself, it is said to be self-excited. Self-excitation consists in the following. In the absence of current in the field coils, the armature rotates in the weak magnetic field produced by the residual magnetism of the pole cores. The weak EMF induced in the armature winding sends a slight current through the field coils. The magnetic field of the field coils builds up, and so does the EMF in the armature winding. This raises the exciting current. This continues until the current in the field coils attains a value corresponding to the resistance of the field circuit. Therefore, self-excitation will take place only if and when (a) the current in the field coils produces a field increasing the field produced by residual magnetism and (b) the resistance of the field circuit does not exceed a definite ( critical) value. Self-excited generators are further subdivided into: (1) shunt-wound units with the field coils connected in parallel with the armature winding; (2) series-wound units; in which the field coils are connected in series with the armature winding; (3) compound-wound machines, in which there are two windings on each pole, one connected in parallel and the other in series with the armature winding. The type of connection of the two windings determines the construction and performance of the generator.
P. Translate the sentences into Russian in writing. Cover the English variants and translate the Russian ones back into English.
It is known to include a wire made of metal with low melting point.
Q. Group work. Make up a number of questions which help you to discuss and revise the main ideas of the Lesson 9.
Chapter LXVI They sauntered along the causeway till they came to the top of the hill on which stood that archway, the memorial to a virtuous widow, which had occupied so large a part of Kitty's impression of the place. It was a symbol, but of what she scarcely knew; she could not tell why it bore a note of so sardonic irony, "Shall we sit down a little? We haven't sat here for ages." The plain was spread before her widely; it was tranquil and serene in the morning light. "It's only a few weeks that I've been here and it seems a lifetime." He did not answer and for a while she allowed her thoughts to wander. She gave a sigh. "Do you think that the soul is immortal?" she asked. He did not seem surprised at the question. "How should I know?" "Just now, when they'd washed Walter, before they put him into the coffin I looked at him. He looked very young. Too young to die. Do you remember that beggar that we saw the first time you took me for a walk? I was frightened not because he was dead, but because he looked as though he'd never been a human being. He was just a dead animal. And now again, with Walter, it looked so like a machine that has run down. That's what is so frightening. And if it is only a machine how futile is all this suffering and the heart pains and the misery!" He did not answer, but his eyes travelled over the landscape at their feet. The wide expanse on that gay and sunny morning filled the heart with exultation. The trim little rice-fields stretched as far as the eye could see and in many of them the blue-clad peasants with their buffaloes were working industriously. It was a peaceful and a happy scene. Kitty broke the silence. "I can't tell you how deeply moved I've been by all I've seen at the convent. They're wonderful, those nuns, they make me feel utterly worthless. They give up everything, their home, their country, love, children, freedom; and all the little things which I sometimes think must be harder still to give up, flowers and green fields, going for a walk on an autumn day, books and music, comfort, everything they give up, everything. And they do it so that they may devote themselves to a life of sacrifice and poverty, obedience, killing work and prayer. To all of them this world is really and truly a place of exile. Life is a cross which they willingly bear, but in their hearts all the time is the desire - oh, it's so much stronger than desire, it's a longing, an eager, passionate longing for the death which shall lead them to life everlasting." Kitty clasped her hands and looked at him with anguish. "Supposing there is no life everlasting? Think what it means if death is really the end of all things. They've given up all for nothing. They've been cheated. They're dupes." Waddington reflected for a little while. "I wonder. I wonder if it matters that what they have aimed at is illusion. Their lives are in themselves beautiful, I have an idea that the only thing which makes it possible to regard this world we live in without disgust is the beauty which now and then men create out of the chaos. The pictures they paint, the music they compose, the books they write, and the lives they lead. Of all these the richest in beauty is the beautiful life. That is the perfect work of art." Kitty sighed. What he said seemed hard. She wanted more. "Have you ever been to a symphony concert?" he continued. "Yes," she smiled. "I know nothing of music, but I'm rather fond of it." "Each member of the orchestra plays his own little instrument, and what do you think he knows of the complicated harmonies which unroll themselves on the indifferent air? He is concerned only with his own small share. But he knows that the symphony is lovely, and though there's none to hear it, it is lovely still, and he is content to play his part." "You spoke of Tao the other day," said Kitty, after a pause. "Tell me what it is." Waddington gave her a little look, hesitated an instant, and then with a faint smile on his comic face answered: "It is the Way and the Waygoer. It is the eternal road along which walk all beings, but no being made it, for itself is being. It is everything and nothing. From it all things spring, all things conform to it, and to it at last all things return. It is a square without angles, a sound which ears cannot hear, and an image without form. It is a vast net and though its meshes are as wide as the sea it lets nothing through. It is the sanctuary where all things find refuge. It is nowhere, but without looking out of the window you may see it. Desire not to desire, it teaches, and leave all things to take their course. He that humbles himself shall be preserved entire. He that bends shall be made straight. Failure is the foundation of success and success is the lurking-place of failure; but who can tell when the turning point will come? He who strives after tenderness can become even as a little child. Gentleness brings victory to him who attacks and safety to him who defends. Mighty is he who conquers himself." "Does it mean anything?" "Sometimes, when I've had half a dozen whiskies and look at the stars, I think perhaps it does." Silence fell upon them, and when it was broken it was again by Kitty. "Tell me, is 'The dog it was that died' a quotation?" Waddington's lips outlined a smile and he was ready with his answer. But perhaps at that moment his sensibilities were abnormally acute. Kitty was not looking at him, but there was something about her expression which made him change his mind. "If it is I don't know it," he answered warily. "Why?" "Nothing. It crossed my mind. It had a familiar ring." There was another silence. "When you were alone with your husband," said Waddington presently, "I had a talk with the regimental surgeon. I thought we ought to have some details." "Well?" "He was in a very hysterical state. I couldn't really quite understand what he meant. So far as I can make out, your husband got infected during the course of experiments he was making." "He was always experimenting. He wasn't really a doctor, he was a bacteriologist; that is why he was so anxious to come here." "But I can't quite make out from the surgeon's statements whether he was infected accidentally or whether he was actually experimenting on himself." Kitty grew very pale. The suggestion made her shudder. Waddington took her hand. "Forgive me for talking about this again," he said gently, "but I thought it might comfort you -I know how frightfully difficult it is on these occasions to say anything that is of the least use - I thought it might mean something to you that Walter died a martyr to science and to his duty." Kitty shrugged her shoulders with a suspicion of impatience. "Walter died of a broken heart," she said. Waddington did not answer. She turned and looked at him slowly. Her face was white and set. "What did he mean by saying 'The dog it was that died'? What is it?" "It's the last line of Goldsmith's Elegy."
Chapter LXVII Next morning Kitty went to the convent. The girl who opened the door seemed surprised to see her and when Kitty had been for a few minutes about her work the Mother Superior came in. She went up to Kitty and took her hand. "I am glad to see you, my dear child. You show a fine courage in coming back here so soon after your great sorrow; and wisdom, for I am sure that a little work will keep you from brooding." Kitty cast down her eyes, reddening a little; she did not want the Mother Superior to see into her heart. "I need not tell you how sincerely all of us here sympathise with you." "You are very kind," whispered Kitty. "We all pray for you constantly and for the soul of him you have lost." Kitty made no reply. The Mother Superior released her hand and in her cool, authoritative tone imposed various tasks upon her. She patted two or three children on the head, gave them her aloof but winning smile, and went about her more pressing affairs.
Date: 2014-12-28; view: 869
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