PROBLEMS. 497 



on account of the fact that a portion of a street, for example, may be much better 

 lighted by a number of distributed glow lamps than by one arc lamp even though the 

 glow lamps aggregate much less spherical-candle power than the arc lamp. 



181. A street is to be lighted for 3,600 hours each year. The 

 street has trees along it so that satisfactory illumination requires, 

 either, (a) one 22 5 -candle-power 3 64- watt enclosed arc lamp 

 every 250 feet, costing $5.25 per year for interest, depreciation, 

 cleaning and trimming, or (<) three 3 2 -candle-power H2-watt 

 glow lamps every 250 feet, each lasting 600 hours, and costing 

 25 cents, including the cost of replacing when burned out. The 

 price paid for power is 3 cents per kilowatt-hour. What is the 

 total annual cost of lighting 250 feet of the street: (a) By en- 

 closed arc lamps, and (&) by glow lamps. Ans. (a) $44.56 ; (b) 

 $40.79- 



Note i. The arc lamp consumes less power here than is specified in problem 

 1 80 inasmuch as street arcs are usually connected in series and operated by a constant- 

 current generator (or transformer) so that there is no ballast resistance in the lamps. 



Note 2. It is very important to consider that a special generator (or transformer) 

 must be used to supply series-connected arc lamps, whereas glow lamps can be oper- 

 ated from the same generator that supplies current for glow lamps for house lighting. 

 Therefore, the cost of operating series-connected arcs is rather more than $44.56 each 

 per year, as compared with $40.79 for three 32-candle-power glow lamps, on account 

 of the interest on the cost and the depreciation of the special generator (or transformer) 

 required in the former case. 



APPENDIX A. MAGNETISM OF IRON. 



182. The intensity of the magnetic field in the air gap between 

 the pole face and the armature core of a dynamo is 3,500 gausses 

 and the field is at right angles to pole face and armature surface. 

 The distance across the air gap is ty& inch. Find the magneto- 

 motive force across the air gap in gilberts and in ampere-turns. 

 Ans. 3,333.5 gilberts, or 2,650 ampere -turns. 



183. Find the magnetomotive force in gilberts and in ampere- 

 turns along a vertical line 10 meters long at a place where the 

 intensity of the earth's magnetic field is 0.56 gauss and its dip is 

 72. Ans. 533 gilberts or 424 ampere-turns. 



32 



