6-A] GENERAL STUDY. 221 



53. Direct Currrent System. A direct current system has the 

 same copper economy as a single-phase system, when the direct cur- 

 rent voltage is made equal to the effective (sq. rt. of mean sq.) value 

 of the alternating voltage. 



If, however, the direct current voltage is increased so as to equal 

 the maximum value of the alternating current voltage, the direct 

 current voltage is increased in the ratio of I \\/2 and the copper is 

 decreased as the inverse square of this ratio. The direct current 

 system then requires only one half the copper of a single-phase or 

 two thirds the copper of a 3-phase system, on the basis of equal volt- 

 age between wires. 



A direct current system would, therefore, be more economical of 

 copper than any other system, at the same voltage. 



54. Choice of Systems. On account of copper economy and the 

 simplicity due to the use of only two wires, direct current would be 

 superior to any alternating current system, if it were not for lack 

 of simple and suitable means for transforming direct current so as 

 to obtain the advantage of high potential transmission with low 

 potential generation and utilization. In the case of alternating cur- 

 rents, these means are provided for by the transformer which makes 

 alternating current systems so flexible that they are practically 

 always* used for long distance transmission, instead of direct current. 



In comparing alternating current transmission systems, the choice 

 is to be made between single-phase with its simpler line construction, 

 fewer insulators, etc. and 3-phase, requiring only 75 per cent, as 

 much copper. If these were all the factors, single-phase transmission 

 systems would be more common than they are, the simplicity offsetting 

 the poorer copper economy. An important and perhaps a determin- 

 ing factor, however, is the superiority of polyphase as compared with 

 single-phase machinery (2) ; for this reason a polyphase system is 

 commonly preferred, quite aside from considerations of copper econ- 

 omy. Of polyphase systems, the 3-phase system is most economical 

 and is therefore the system in general use. 



* (54a). In a few cases high potential direct current has been used for 

 power transmission, notably in the Thury system. This is essentially a 

 constant current system. The high potential is obtained by generators in 

 series; the motors are likewise in series. See Land. Electrician, March 

 19, 1897; New York Elect. Rev., January, 1901. 



