XXVIII. W. H. WARREN. 
require one-fourth as much copper to transmit a certain amount 
of energy if 2,000 volts be used, as will be required to transmit 
the same energy with equal efficiency over the same distance 
if 1,000 volts be employed. And it will require yoo 48 
much copper if 10,000 volts be used as will be necessary if the 
potential is limited to 1,000 volts. Hence the necessity of using 
high voltages in long distance transmissions, and this is most 
economically accomplished by the use of alternate current 
generators and transformers. The great advantage of the 
alternating current generators over the direct current generators 
is that no commutator is necessary. That part of a direct current 
machine which is especially liable to cause trouble when high 
potentials are employed is the commutator, which is of all 
essential elements of a generator or motor the most complex and 
expensive, while it is at the same time the most delicate and 
liable to damage. Again, in the alternating current machine, the 
manner of placing the wires on the armature admits of better 
protection, more thorough insulation, and greater facility of repair, 
since one wire is wound continuously and there is no ex 
part, except the collector rings to which ends of the wire are 
connected. The necessity of breaking the windings of a direct 
current machine into so many parts makes thorough insulation 
of the armature very difficult. 
The Westinghouse Company have developed two distinct 
systems for transmitting alternative currents, viz., the syt- 
chronous or two wire alternating current system, and the Tesla 
polyphase or multiphase system. 
In the two wire synchronous system the generator and motor 
are connected with two wires, and but one current flows betwee? 
them. In the polyphase system the generator and motor are 
connected with three or more wires, and two or more currents 
differing in their time relation or phase traverse the wires. 
In the two wire system the number of alternations which are 
employed is about 7,200 per minute ; in the multiphase syste™ 
# lower number is ade The 60. wire systems are not self- 
