CHAPTER XIII 



ELECTRIC CURRENTS AND THEIR MEASUREMENT; 

 ARC LAMPS, THEIR WIRING AND CONTROL; 

 CANDLE-POWER OF ARC LAMPS FOR 

 PROJECTION 



650. Apparatus and Material for Chapter XIII: 

 Direct and alternating current ; Voltmeter, ammeter and watt- 

 meter for direct and alternating currents ; Shunt generator, motor- 

 generator set, mercury arc rectifier ; Polarity indicators ; Arc lamp ; 

 Rheostat, inductor, transformer and other ballast; Carbons; 

 Water-cell; Insulated wires, flexible cables, insulators, switches, 

 separable plugs, caps, etc. ; Fuses and circuit breakers; Wire, iron 

 plates, etc., for home-made rheostats. 



651. Historical development of electric lighting. See the 

 Appendix. 



ELECTRIC CURRENTS: KINDS AND COMPARISON 



652. Direct current. The earliest electric currents studied 

 and experimented with were produced by the voltaic cell. These 

 currents have a constant polarity, always flowing in the same 

 direction. 



A direct electric current may be produced by a voltaic battery, 

 or by a dynamo ( 652a). 



The first installations of electric plants were all for direct current, 

 now they are mostly for alternating current (see below). 



The principal use of direct current at present is for trolley cars, 

 and other apparatus where variable speed motors are necessary; 

 in electrolysis such as charging storage batteries and the decomposi- 

 tion of chemical compounds; for electric lighting in some of the 

 more densely populated cities and for projection purposes. 



652a. Generator, Dynamo. Generator is a comprehensive term in- 

 cluding all means of producing electric currents whether these means be 

 chemical or mechanical. 



Dynamo, on the other hand, is a special term denoting a generator by which 

 mechanical is transformed into electrical energy; for example, a steam engine 

 may give motion to the dynamo and thus produce an electric current. In a 

 word, a dynamo is a generator of electricity, but all generators of electricity are 

 not dynamos. 



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