Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . 28 .48 29 .38 30 .30 31 .24 32 .19 33 .15 .8015 34 .12 .6354 35 .10 .5039 104; 36 .08 .3997 37 .06 .3170 38 .05 .2513 39 .04 .1993
Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . 28 .48 29 .38 30 .30 31 .24 32 .19 33 .15 .8015 34 .12 .6354 35 .10 .5039 104; 36 .08 .3997 37 .06 .3170 38 .05 .2513 39 .04 .1993 40 .03 .1580 *1 mile pure copper wire ohms at ° C. or 1-16 in. diam. ~ * F. 1 circular mil is .7854 square mil. CHAPTER III Current Generation in Dynamos — Dynamos — Brushes andCommutators. Current Generation in Dynamos. If we take a coil ofwire, Fig. 15, and rapidly thrust a magnet into it, weshall observe a certain deflection of the galvano-meter needle shown with it. This deflection con-tinues only while the magnet is in motion. After. FIGURE 15. we have inserted the magnet and it has come torest the galvanometer needle will return to its normalposition. When we withdraw the magnet the deflec-tion of the needle will be in the opposite direc-tion. If the magnet is inserted or withdrawn witha veiy quick motion, the deflection will be consider-able. If the magnet is very slowly inserted or with-drawn the deflection will hardly be noticeable. The40 ELECTRICITY FOR ENGINEERS 41 same phenomena will occur if instead of moving themagnet, we hold it stationary and move the coil, or ifboth of them be moved towards or from each deflection of the compass needle indicates that acurrent of electricity is passing along the wire, and theexperiments above described show exactly how cur-rents of electricity are produced in dynamos. An electromotive force is induced by rapidly cutting lines of force, that is, by moving either a
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