216 ELECTRICAL ENERGY. 



decreased amount of copper as compared with the alternating systems, 

 and in the absence of induction effects, which are a drawback to alter- 

 nating-current transmission. 



TRANSMISSION BY ALTERNATING CURRENTS. 



A large j>roportion of the transmission plants that have been installed 

 in the last few years have been of the alternating current type. These 

 have, as a rule, given satisfactory results, and the installations that are 

 now being erected or planned are almost exclusively on an alternating 

 current basis. The great advantage of this system lies in the fact that 

 it is possible to change the voltage of the current without the use of 

 rotating apparatus, and at once economically and safely. Low voltage 

 dynamos may be used, the voltage may be increased in any desired 

 ratio by stationary transformers, the energy may be transmitted at 

 an increased voltage, and at the receiving end the voltage may again 

 be reduced by transformers. If we compare this method with the 

 continuous current system we will see that to obtain an alternating 

 current of the required pressure at the receiving end of the line we 

 would use the same number of transformations required by the contin- 

 uous current system. We have the great advantage, however, that our 

 changes in voltage have been obtained by the agency of stationary 

 apparatus, which is much cheaper, is more efficient, and is safer than 

 that required in the continuous current system. It is possible to 

 increase the voltage by means of transformers to almost any value with 

 perfect safety and with an efficiency as high as 98 per cent or 99 per 

 cent. If, then, our alternating current, when it has been reduced at the 

 receiving end, is as valuable for distribution as the current obtained by 

 the direct current system, there will be no doubt that alternating trans- 

 mission has great advantages over continuous currents. 



I have spoken of the relative amounts of copper required by the 

 single-phase, two-phase, and three-phase alternating currents. I do 

 not think it necessary to explain minutely the difference between these 

 systems, as they are well understood. In a single-phase system a single 

 alternating current is used. In a two-phase system two alternating 

 currents, whose phases differ by 90 degrees, are employed, while in the 

 three-phase system there are three currents, differing in phases by 60 

 degrees. I shall consider the characteristics of these three systems, as 

 there has been much discussion, especially as to the relative value of 

 the last two of them, for transmission work. I shall not discuss the 

 various modifications of the systems, but shall confine myself to general 

 considerations. There is no single-phase motor in successful commercial 

 operation that does not require to be started from rest by some outside 

 means. This prevents a single phase current from being used at the 

 present time for power distribution; and as ? in most transmission, the 

 distribution of power is an important item, single-phase currents are 

 not suitable for this purpose. In a two-phase system the currents are 

 usually carried on separate pairs of wires, while in the three-phase 

 system three wires are generally used, a common return being unneces- 



