ELECTRIC TRUNK-LINE OPERATION—SPRAGUE. ° 143 
urban roads, is unfitted for trunk-line operation, and where overhead 
trolley wires are used the long collector will probably take its place. 
Physically, the overhead trolley is not individual to any particular 
system; practically, its use depends upon the amount of current 
which has to be collected. If at low potentials, then it must be either 
strongly reinforced, and there must be a plurality of contacts, or if 
these are diminished the amount of current to be collected must like- 
wise be reduced, and the potential raised. In the abstract, therefore, 
the possible use of the overhead trolley, no matter by what system, is 
a question of allowable operative potential, and the amount of current 
which can be practically collected. 
Until recently the invariable practice with overhead construction 
has been to use a flexible wire supported at comparatively long dis- 
tances on tangents, with pull-off at curves, and easily yielding to the 
pressure of a trailing trolley. This is the practice which charac- 
terizes not only direct-current trolley operation, but has also distin- 
guished practically all operation abroad where single-phase or 
polyphase currents have been used. The introduction of high ten- 
sions has, however, now made it necessary to provide by additional 
supports against the possible breakage of the trolley wire. This has 
led to the introduction of catenary construction, the catenary being 
either single and supporting the trolley at frequent intervals, or 
double to prevent lateral swaying. In the former case the trolley 
is only partly flexible, and in some cases the support has been sup- 
plemented by an intermediate catenary, as on the Blankenese- 
Ohlsdorf Railway, where greater flexibility of the trolley wire itself 
-is insured by loosely suspending it from the lower member of the 
catenary instead of making the latter the trolley wire, and providing 
for varying the tension. In other cases it is to some extent main- 
tained by having less frequent supports, and also by introducing a 
movable part at the suspender. 
The most recent and extended application of double-catenary con- 
struction is that on the New Haven road for use with its single-phase 
locomotives (pl. 11). Here a trolley wire is put under high tension 
and is supported at frequent intervals by solid clips attached to rigid 
triangles, in turn secured to galvanized-iron wire cables carried on 
insulators on the top of bridges which span the tracks at intervals 
on tangents of about 300 feet. The catenaries are drawn together 
between the spans so as to give the utmost rigidity to the whole sys- 
tem, the intent being to maintain the trolley wire as nearly as possi- 
ble in one plane. At cross-overs and sidings the supporting triangles 
overlap, and the angle between the junction and the trolley wires is 
filled with additional conductors, more readily to insure safe passage 
of the vertically moving sliding contact which has been adopted. 
At intervals of about 2 miles the trolleys are sectionalized at anchor 
