Electricity for public schools and colleges . onversion of the readings into volts under all conditions of sensitive-ness are sent with each instrument. The terminals are of an ingenious construction, designed partly toobviate accidents in measuring very large , and the leads areso arranged that the current passing through them has no action onthe needle. § 14. Sir W. Thomsons Graded Current Galvanometer.—Inthe construction of the last instrument it was desired to measure CH. xvn. GALVANOMETERS J9T the -^V between two points without appreciably diminishing thisJiV by lessening the resistan
Electricity for public schools and colleges . onversion of the readings into volts under all conditions of sensitive-ness are sent with each instrument. The terminals are of an ingenious construction, designed partly toobviate accidents in measuring very large , and the leads areso arranged that the current passing through them has no action onthe needle. § 14. Sir W. Thomsons Graded Current Galvanometer.—Inthe construction of the last instrument it was desired to measure CH. xvn. GALVANOMETERS J9T the -^V between two points without appreciably diminishing thisJiV by lessening the resistance between them. Hence we sawthat the coil was made of a very high resistance. In the present case it is desired to measure currents withoutappreciably diminishing them by the introduction of coil, therefore, must be of as low a resistance as is consistentwith the requisite sensitiveness. The citrrent galvanometer, or am-meter, is in general constructionalmost identical with the last. But the coil is composed of a few turns. of thick copper wire, or copper strip, of a resistance equal to aboutj~yp ohm. There is also a special arrangement of the terminals, toobviate inconveniences or accidents in dealing with large currents, andthe leads are so arranged that the current passing through them hasno action on the needle. § 15. Webers Electro-Dynamometer. It is sometimes desired to measure currents by some instrumentthat does not depend upon the magnitude of the earths field of forceat any particular place—in fact, to construct an instrument that can beused without reference to the variable quantity H, that has appearedin the formuke of the preceding instruments. Now we shall see in later Chapters that a coil carrying a currentacts as a magnet of the same shape whose poles answer to the twofaces of the coil. It is found that the magnetic moment of such a coildepends solely upon its shape and the strength of the current flowingthrough it; so that, as long as thes
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Keywords: ., bookcentury1800, bookdecade1890, bookpubl, booksubjectelectricity
