Page 434 : COMMONS, HOUSE OF — COMMUTATOR
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cians, however, a nation of traders whose home was on the strip of coast that lies about Tyre and Sidon, were the first merchants to plan and execute great voyages by sea for the sake of trade. Their ships roved even to the Atlantic and the distant shores of Britain. The Greeks also were great traders; and Carthage, a Phoenician colony, became so powerful and wealthy through her commerce that it required all the might of Rome to humble her in the dust. In the middle ages the great trade between Asia and western Europe lay chiefly in the hands of the merchant republics of Venice and Genoa. But so soon as the route round the Cape of Good Hope to India was found and the new continent of America also came to attract the eye of the European trader, commerce centered in the nations of the Atlantic rather than of the Mediterranean. At first Spain and Portugal, and afterwards England and Holland became the great world-traders. In the latter part of the long strife of England and France for colonial empire the United States, having won their independence, became the great neutral carriers of merchandise.
The extent to which commerce grew in the nineteenth century is almost beyond belief. It was favored greatly by the invention of the telegraph, the success of ocean-cables (q. v.), the building of railroads and the invention of steamships. It was only in 1819 that the first steam-vessel crossed the Atlantic Ocean. In England in 1800 the imports were valued at under $29,000,000; in 1900 at over $523,000,000; while the exports in 1800 were valued at under $35,000,000; and in 1900 at over $354,000,000. Or take the commerce of the United States in the second half of the nineteenth century. Whereas in 1850 the exports and imports together amounted to $318,000,000; in 1900 they amounted to no less than $2,244,000,000 in estimated values, while in 1907 they amounted to $3,315,272,503.
Commons, House of (English). See Parliament.
Com′mune of Paris (kŏm′ mūn). Commune is the unit or lowest political division of France, corresponding to the American township. The rising of the Commune of Paris, in 1871, should not be confounded with communism, with which it had nothing to do. It was a revolutionary attempt to establish self-government for Paris. The theory of the rising was that every commune should have self-government and that the state or central government should be merely a federation of communes. The movement was caused by discontent in Paris, where the people found themselves in possession of arms after the siege by the Germans. The rising occurred on March 18, 1871, and was put down only after 10 weeks’ long and bloody fighting between the forces of the commune and a large army of the central government. Sixty-five thousand communists fell during the last ten days in May.
Com′mutator. In most kinds of electrical work it is necessary at times to change the direction of the current in some part of the circuit. The commutator is an instrument for producing this change. The device represented in the accompanying figure is typical of all commutators. The block A, B, C, D is made of wood, and has four holes bored about half-way through it. These holes, placed one at each corner of a square, act as cups to hold mercury.
Into the sides of the block are inserted four wires, E, F, G, H. Each of these wires connects with the mercury in the cup nearest it. The poles of the battery are joined to two diametrically opposite cups, say B and D; the wires from the rest of the circuit, say from a galvanometer, are joined to the remaining two cups, A and C. For closing the circuit two short, thick, copper conductors P and Q, are mounted on a movable block, as shown in the figure. These connectors, P and Q, may be placed in the mercury cups in such a way as to connect B with A, and hence C with D; or they may be placed so as to connect B with C, and hence A with D. In the one case the current flows from G to E through the galvanometer; in the other case from E to G. To change the direction of the current in the galvanometer circuit, we have, therefore, only to lift the top block and rotate it through 90° in either direction. Such an instrument is called a commutator. In many kinds of electrical machinery the commutator is made to work automatically. Thus, in the case of the dynamo, the currents which are generated in the coils of the armature are alternating, but a commutator is in many instances placed on the shaft of the armature, which automatically reverses the connections of the armature-coil and the external circuit, so that the current in the external circuit always flows in one direction. Such a dynamo is called a direct-current generator. The dynamo which is not provided with a commutator is called an alternate-current generator.
Image: A TYPICAL COMMUTATOR
