The practical telephone handbook and guide to the telephonic exchange (1906) (14569551630)

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:

attery or cell is joined to a known resistance—a, includingresistance of ammeter, which, however, may usually beneglected. Then if r be the resistance required, by Ohms E E law c = ——. From this 0 (r + a) = El5 or r = ^-a (1).r + a v J l5 c w Also as the difference of voltage between open and closed circuit = Ex — E2 is due to loss in forcing the current through the internal resistance, r also = -± -. (2). For example, if 470 PRACTICAL TELEPHONE HANDBOOK e,x = 1*37 volts, e2 = 81, c = 8 amperes and a = lOl ohms, then by (1) r-i^-lOl = 1*71-lOl = *7 ohm, and by (2) T37 — = —— — 1 ohm as before. The above formulae apply not only to batteries, but to anycircuit or part of a circuit. If direct measurement instruments are not available, themost common method is to connect up the cell by short andthick wires to a reflecting galvanometer, shunted with a pieceof wire whose resistance may be neglected. A convenientdeflection being obtained by varying the length of wire forming

Text Appearing After Image:

Fig. 446. -Connections of Wheatstone Bridge for Test of BatteryResistance the shunt, a resistance of, say, 1 ohm is unplugged in a resis-tance-box included in the circuit, and the reduced deflectionobserved. Fig. 446 shows the connections using a Post-Officebridge. The deflections in the two cases will be proportionalto the strength of the currents producing thero, and, therefore,inversely proportional to the resistances in circuit in the twocases, which will be r and r +1 respectively, if r = the internalresistance of the cell. Suppose the deflections are 160° and50°, then r + 1 : r = 160 : 50, or 50 + 50 = 160r, from which r = —— = 455 ohm. The defect of the above methods is that the E.M.F. mayvary during the two tests, the current being stronger in thefirst case, and the polarisation consequently greater. Current Strength,—In the absence of an ammeter, the ELECTRICAL MEASUREMENTS 471 strength of the current passing through a conductor is deter-mined by finding the total res

the practical telephone handbook and guide to the telephonic exchange 1906 book illustrations telephone images from internet archive

1906

Northeastern University, Snell Library

http://commons.wikimedia.org/

public domain