160 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
the added very great practical advantage that the generators not 
only work at lower potentials, but on closed metallic circuits, are 
removed from direct contact with working conductors and earth, and 
have interposed between them and the line at least one set of static 
transformers, which practical experience has shown to be one of the 
best generator safeguards against lightning, and which, if broken 
down, do not involve large and costly units nor wholesale sacrifice of 
capacity on roads where the adoption of electricity is warranted. 
General cost comparison of direct-current and single-phase alter- 
nating-current systems.—l have made many comparative analyses, 
involving millions of dollars, and I have found that where equal 
permanence of installation 1s provided for, and equal ultimate as well 
as average duty, there is not on demonstrated facts a wide variation 
in the initial cost of plant. 
Ordinarily, the signal systems used on railroads will have to be 
changed at considerable cost. Fortunately, methods have been de- 
veloped which permit the use of all the rails for the main return 
circuit by using a special alternating-current circuit for operating 
the signals. Where the tracks are used for direct-current return, 
reactance bonds are inserted which permit the flow of the direct 
current, but resist that of the alternating signal current. Where 
the tracks are used for alternating-current operation, and are like- 
wise subject to the flow of direct currents, the signals must be oper- 
ated by alternating currents of high frequency through apparatus 
which is inoperative to currents of low frequency or to continuous 
currents. 
Field of the single-phase alternating-current motor.—lt would be 
idle to deny, and I have no wish to belittle the good work done and 
the results achieved in the development of the single-phase motor, 
just as it would be equally unwise to ignore what has been done in 
polyphase and direct-current work. It seems to me that the present 
principal field of usefulness of the single-phase system is on roads of 
considerable extent which operate an irregular and sparse traffic, and 
where only a moderately expensive, or what may be called a second- 
class overhead construction, which will keep down the ratio of line 
investment to that of the balance of equipment, is tolerable. As one 
departs from this condition, adopts more permanent construction, 
and faces the problems of denser traflics and higher capacities, any 
advantages of the single-phase system disappear, and the superiority 
of the direct-current equipment, with such improvements as are in 
sight, becomes manifest. But whatever may be the future of single- 
phase operation under the conditions stated, any present claim for it 
as the preferable equipment for congested service demanding high 
schedules and great capacity is not worth a moment’s thought, for in 
this field it can not touch the direct-current system. 
