ELECTRICAL ENERGY. 215 



possesses some advantages and some disadvantages. If we consider 

 tlie relative cost of the copper in the line for a given amount of power 

 transmitted and for a given maximum potential between the conduct- 

 ors, we will find that the relative amounts for the continuous current 

 and the different alternating-current systems will be as follows: 



Continuous current 100 



Single-phase alternating 200 



Two-phase alternating 200 



Three-phase alternating 150 



We see, then, that the continuous-current has a marked advantage 

 over the alternating-current system as far as the cost of copper is con- 

 cerned. There are, however, certain practical disadvantages belonging 

 to this system. The high voltages necessary for long-distance trans- 

 mission makes it impossible to distribute the current at the receiving 

 end without first reducing the voltage. With continuous current this 

 can only be done by employing a rotary commutator of some kind. A 

 plan which has been practically and successfully used has been to run 

 a number of dynamos in series at the generating end of a line, while 

 at the receiving end are a number of motors, also arranged in series, 

 which are used to drive other generators to give the required type of 

 current and the desired voltage. It has not been found possible to 

 make either dynamos or motors of any great output, as there are prac- 

 tical difficulties in running dynamos of high potential where the current 

 taken from them has a considerable value. Mons. Thury has installed 

 a number of continuous-current transmission plants that have appar- 

 ently given excellent results. At Biberist a transmission of 15 miles is 

 employed. At Brescia 700 horsepower are transmitted over 12 miles at 

 a maximum of 15,000 volts. Mons. Thury states that generators for 45 

 amperes can be constructed up to 3,000 volts, and he thinks that 4,000 

 could be successfully used. These machines, however, are small when 

 compared with the 5,000 horsepower dynamos in use at Niagara, for 

 instance; and where the transmission is a large one the great number 

 of machines necessary would be a serious objection to this type of trans- 

 mission. It will be seen that the greatest possibility of trouble in such 

 a transmission lies at the ends of the line, in the generating and receiv- 

 ing apparatus. It is necessary, no matter what our voltage is, that both 

 the dynamos and motors shall be directly subjected to it, and this with 

 commutated machines will always be a source of danger. If we are to 

 do any considerable amount of lighting from such a station, our energy 

 for this purpose undergoes three transformations before it reaches the 

 lamps, and the efficiency would not be so high as in a corresponding 

 alternating-current system. It would hardly be possible to supply 

 motors for ordinary work at the high voltages used for transmission, 

 and the current for them would have to be transformed in the same 

 manner as the current for the lamps. It must be recognized, however, 

 that this system has been successfully used and has given excellent 

 results in a few cases of transmission. Its great advantage lies in the 



