GENERATING ELECTRICITY COMMERCIALLY 201 



power furnished by the dynamo by the amount of mechanical 

 power delivered to the dynamo. It is measured by indicat- 

 ing the engine while running the dynamo at full load and 

 noting the reading of the ammeter and voltmeter, and then 

 indicating the engine when the dynamo is idle. The differ- 

 ence between the two readings is approximately the mechani- 

 cal power supplied to the dynamo. 



Watts Volts X Amperes 



= Horse-Power 



746 746 



Motors are rated in horse-power (H.P.) Dynamos are 

 rated in kilowatts (kw.). 



1 H. P. = % kw. 



247. Electric Transformers. The commercial require- 

 ments of users of electricity are best served by distributing 

 electricity at high voltage and low 



amperage and by changing the 

 same current into low voltage 

 and high amperage by means of 

 transformer placed on a pole, or 

 better in a vault, before the elec- 

 tricity enters the building. 



. ; ,. _ m . , FIG. 92. A Transformer. 



A transformer (Fig. 92) consists 



of three parts: (1) the primary coil, which is the wire which 

 connects with the alternating current from the supply lines; 

 (2) a core of iron; (3) and a secondary coil or wire in which 

 is generated an electromotive force by the change of magnetism 

 in the core which it surrounds. 



248. Fuse. A fuse is a safety device intended to melt 

 when a current exceeding a certain strength passes through 



