ELECTRICAL ADVANCE IN THE PAST TEN YEARS. 127 



with a few electric motors. Here and there, isolated lighting plants in 

 mills and other large buildings were in operation; but the alternating 

 current, so large a factor in electrical enterprises nowadays, had scarcely 

 become known or applied practically. There were perhaps not more 

 than twenty trolley cars in actual service in 1887, and these were of 

 doubtful success. There were no regularly constituted electric rail- 

 ways worthy of the name. The telephone and electric-lighting wires 

 were largely overhead, and frequently the construction was of the most 

 imperfect and temporary character. Among some notable exceptions 

 stood prominent the Edison three- wire underground vsystem, which had 

 the elements of permanence. The extensive underground mains and 

 wires in use in cities to-day testify to the great progress which has 

 taken place in the means of distributing electric energy. They repre- 

 sent a very large investment of capital, but they also confer that reli- 

 ability and permanence which was before lacking. 



Within the past eight or ten years much has been done in the perfec- 

 tion of thoroughly practical forms of meters and other instruments for 

 the measurement of electric forces and quantities. While such work 

 resembles in its delicacy that demanded by watch mechanism, on the 

 other hand the large station dynamos are examples of the heaviest 

 machine construction. Some of them demand steel castings more than 

 30,000 pounds in weight. Indeed, in the same electric iactory we may 

 find watchmaking tools turning out the fine pieces of electric meters, 

 which may not weigh more than a few grains, and electric cranes han- 

 dling masses of metal of many tons — parts of the larger dynamos under 

 construction. A few years ago a dynamo was large if it demanded 100 

 or 200 horsepower to drive it, while now such jnachines are diminutive 

 when compared with those of 2,000 horsepower commonly constructed. 



Dynamos are in use at Niagara of 5,000 horsepower capacity. A 

 single one of these would supply more than 50,000 incandescent lights 

 such as are ordinarily used, or would give motion to 500 trolley cars. 



The i:>eriod since 1887 has been marked by great extension in electric 

 lighting by both arc and incandescent lamps. Prior to that year only 

 the largest cities, broadly speaking, possessed any electric-lighting 

 service. Now, however, even the smaller towns have their electric sta- 

 tions, their arc lamps for street lighting, and the smaller incandescents 

 for general use. The same wires or mains frequently supply both kinds 

 of lights. The incandescent lamps in use in the United States are num- 

 bered by millions, and there are several hundred thousand arc lamps 

 beside. There are in operation nearly 3,00iJ electriclight-supiDly sta- 

 tions, and these, together with isolated electric plants, represent a 

 capital of about $500,000,000, 



One of the chief factors in this great extension has been the applica- 

 tion of alternating electric currents, or currents of wave-like natare, 

 reversing their direction many times in each second. The direct or 

 continuous current had previou.sly occupied the field alone. But the 



