File:An elementary book on electricity and magnetism and their applications (1919) (14781485474).jpg

Identifier: elementarybookon00jack (find matches)
Title: An elementary book on electricity and magnetism and their applications
Year: 1919 (1910s)
Authors: Jackson, Dugald C. (Dugald Caleb), 1865-1951 Jackson, John Price, b. 1868 Black, Newton Henry, b. 1874
Subjects: Electricity Magnetism
Publisher: New York, The Macmillian Co
Contributing Library: Internet Archive
Digitizing Sponsor: Internet Archive

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Fig. 61. — Telegraph relay. many turns of fine wire on their magnets. Thus they may besatisfactorily operated on as little current as eight or tenmilliamperes.* These in-struments are called relays(Fig. 61). Reading signals directlyfrom a relay is not usuallyattempted, as the motionof its armature is so deli-cate that it makes verylittle sound, but the arma-ture and one of its stopsare arranged as a part of a local circuit which contains a sounderand a couple of gravity cells (Fig. 62). As the relay armature moves back and Line forth it makesand breaks thelocal circuit andreproduces in itthe signals whichpass over themain line. Thesounder in thelocal circuit givesthe signals ex-actly as theypass over theline. The tele-graph circuitsused in thiscount ry are keptclosed when not in use. Therefore closed-circuit cells,that is, cells with continuous depolarizing qualities, must be * A milliampere is one one-thousandth of an ampere.
Text Appearing After Image:
Local Battery Fig. 62. —Apparatus at telegraph station, including key,relay, and local circuit, with battery and sounder. 104 ELECTRICITY AND MAGNETISM used to supply the current; and the key is provided with ashort-circuited switch (Fig. 59), which keeps the circuit closedwhen the key is not in use. When telegraphing, the switchis open and the key makes and breaks the circuit as it ispressed and released by the fingers of the operator. Questions 1. An electromagnet is found to be too weak for the purpose in-tended. How may its strength be increased? 2. In looking at the AT-end of an electromagnet, in which directiondoes the current go around the core, clockwise or anti-clockwise ? 3. If you find that the .V-pole of a compass held under a north andsouth trolley wire points toward the east, what is the direction of thecurrent in the wire? 4. A feed wire for an overhead trolley line is conducted up a verticalwooden pole from an underground duct. If one approaches the polefrom the sout

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