The practical telephone handbook and guide to the telephonic exchange (1906) (14569789277)

Identifier: practicaltelepho00pool (find matches)

Title: The practical telephone handbook and guide to the telephonic exchange

Year: 1906 (1900s)

Authors: Poole, Joseph

Subjects: Telephone

Publisher: New York, Macmillan Co.

Contributing Library: Northeastern University, Snell Library

Digitizing Sponsor: Boston Library Consortium Member Libraries

Text Appearing Before Image:

ELECTRICAL MEASUREMENTS 453 direct to the proper point on the scale without unnecessaryvibration about that point) and its freedom from the effectsof external vibration and magnetic influences. It is not,however, so sensitive, and has not the all-round usefulnessof the Kelvin instrument. Fig. 428 shows its construction.Between the poles of a strong compound horseshoe magnetis fixed a soft-iron cylinder, which serves to direct the lines offorce, and in the space between this and the poles of themagnet a rectangular coil of wire, with mirror attached, issuspended by a very thin silver wire at top and bottom, thes e

Text Appearing After Image:

Fig. 428.—DArsonval Galvanometer wires also serving as connections to the coil. A spring atthe bottom and torsion screw at top serve to adjust. When a current passes through the coil the latter is de-flected to one side or the other, the coil tending to set itselfat right angles to the line joining the poles, so as to embracethe greatest number of lines of force, but having its deflectionrestricted by the torsion of the suspending wire. The figureshows the instrument with its cover removed. The coil isusually wound to about 750 ohms. Shunts.—As these reflecting instruments by themselveswould be much too sensitive for many purposes, a set of 454 PRACTICAL TELEPHONE HANDBOOK shunts is used with them, by means of which the sensitivenessmay be reduced to T\, ^, or ^\m by diverting ^, T%\, or tVo9o °t the current fromthe galvanometer. Aplan of the top of sucha shunt-box is shown in Fig. 429. The shunts are markedh A> and 9J9-, thesebeing the fractions of theresistance of the gal-va

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1906

Northeastern University, Snell Library

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public domain