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The measurement of electric currents


The measurement of electric currents

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    Available in PDF Format | The measurement of electric currents.pdf | Unknown
    Thomas Commerford Martin
Excerpt from book: ami with the magnetic field arranged so as to give readings through a large angle. The Weber dynamometer still survives in the Siemens dynamometer, and in Lord Kelvin's balances. The Siemens dynamometer, Fig. 19, consists of a fixed Fig. 18. Fig. 19. and a moving coil, r and o, suspended :it right angles. The moving coil tends to arrange itself parallel to the fixed coil, and the force is resisted by a spring and milled head. The contacts to the moving coil are made by mercury cups, s s.This instrument is not direct-reading, and is influenced by external fields, but is still largely used in alternate current work. The Kelvin balances are, really, carefully made scales, for weighing the force exerted by coils carrying currents. The principle is shown in Fig. 20. The great difliculty in such instruments, especially for use as amperemeters, is to make the connections without interfering with the accuracy of the balance. Lord Kelvin has overcome Fig. 20. this, by suspending the moving system by a number of fine wires. These can carry large currents, and are much more flexible than a single thin strip. These instruments are not direct-reading, but that is a small matter, as they are intended not as station or installation instruments, but as secondary standards for those who have no standard cell gear. Some forms are, however, made direct-reading, for engineroom use. The beam t is supported by the ligament u, made up of fine wires : v v are the coils. The next form depends on the heating of a wire by the current in it. Fig. 21 shows the first practical form of this instrument, due to Mr. G. M. Hopkin. Two Fig. 21. .vires w x of the same material are used. The expansion of one, x, moves an index, and this wire carries the current. The 3ther, w, is arranged so that i...  
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