142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
Comparison of direct-current and alternating-current braking.— 
On the general subject of braking it should be pointed out that with 
direct or continuous current motors there is always a residual magnet- 
ism in the fields because of their construction, and the fact that the 
exciting current never changes direction. Such machines, therefore, 
ean always promptly build up automatically when properly closed 
upon themselves, and the reverser is set in the proper direction. 
A similarly effective method of braking has been claimed for 
motors operated by single-phase alternating currents, but it would 
seem that in this case there is not the same degree of reliability. In 
such motors the field is laminated to the last degree to cut down 
heat losses, and to increase the capacity; it will hold but little resid- 
ual magnetism under any circumstances, and furthermore the field is 
excited by a rapidly varying alternating current. It is therefore pos- 
sible that at times the field will be nearly inert, and comparatively 
slow, with its low-turn winding, in building up, or possibly the 
field may be entirely inert, and may refuse to build up at all. There 
seems, therefore, no certainty whatever that a single-phase alternat- 
ing-current motor, disconnected from the line, and without any other 
exciting source, will, when closed upon itself, always build up into a 
braking dynamo. 
All things considered, reliability and simplicity of operation dic- 
tate the use of the self-exciting method of braking with the direct- 
current motor, which lends itself to that purpose in the highest 
degree. — 
Working conductors——Whatever motors are used—and all the 
principal types will be used—there are various methods of con- 
struction and use, especially as applied to locomotive building, and 
alternate methods of current supply and use. 
Generally speaking, conductors may be divided into two classes: 
Flexible or rigid overhead, and third-rail. One would suppose from 
many references and some of the arguments which have been made, 
that direct-current systems are essentially and necessarily dependent 
upon the third-rail, and that the overhead trolley is a thing individ- 
ual to, and has been developed for, alternating-current operation only. 
This impression should be corrected, not of course for the information 
of engineers, but because this somewhat erroneous idea is in danger 
of being accepted as a fact by non-technical men. 
The overhead system has been a distinctive feature of all electric 
roads operated by direct current since the days of the historic Rich- 
mond road, with the exception of those using the third rail; and until 
recently the only practical modification has been the somewhat lim- 
ited use abroad of the sliding bow or roller in place of the grooved 
trolley wheel. This latter, although used with high speeds on inter- 
