144 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
bridges, where are provided the necessary switches for cutting out 
sections, and for looping to extra supply conductors. 
The modern pantograph consists of a sliding or rolling contact, 
which forms the upper number of a light yet strong collapsible struc- 
ture maintaining an upward spring pressure. The theory of this 
system of collection is that a locomotive normally moves between two 
parallel planes, on one of which it runs and from the other of which 
it collects current, and that the ordinary motion of the contact will be 
inappreciable. This assumption is, however, modified in practice. 
-The collectors are carried normally 22 feet above the track on a super- 
structure (pl. 11) which must respond in some measure to track irregu- 
larities, and which has considerable inertia and some friction. There 
is a drag because of friction against the trolley wires and wind pres- 
sure due to motion of the locomotive. This upward pressure must 
necessarily be changeable because of variation of angle, friction, and 
the resultant motion. To maintain contact it must rise and fall. 
When traveling 70 miles an hour it passes supports which are more or 
less rigid nine times a second, and between these supports the trolley 
wire, no matter what the tension, will be convexed upwards. As the 
collector approaches any suspender the pressure will normally con- 
siderably increase, and, as it leaves it, diminish. The practical ques- 
tion arises whether, considering all the forces acting on it and its 
inertia, it can satisfactorily respond in addition to other requirements 
to a double change in vertical direction nine times a second. If con- 
tact depended upon the whole structure of the pantograph moving 
thus rapidly some trouble might be anticipated, but possibly the elas- 
ticity of the upper part will prove sufficient. 
An ingenious method of making contact with an overhead single 
trolley line is that developed by the Oerlikon Company under the 
direction of Mr. Huber (pl. tv). In this system the trolley is 
stretched with comparative rigidity on top of insulators supported 
on posts alongside the track, with cross-overs where needed. In place 
of the ordinary wheel and bow trolleys, a curved hinged arm of fair 
length, and sweeping over nearly one half a circle in a plane trans- 
verse to the line of track, is supported on insulators on the side of the 
car. Normally, this bow rests on top of the wire, pressing lightly 
on it, and thus avoiding the under formation of icicles. On cross- 
overs and in tunnels, where the trolley wire is carried over the track, 
the arm swings toward the center of the car, and is depressed, making 
contact progressively from the top around to the side, and then under- 
neath the trolley wire. In addition, the saddle which carries the bow 
is movable laterally, increasing the radius of action. Of course two 
bows can be used. 
The alternative type of working conductor is the third rail, already 
adopted on about forty roads, some of considerable extent, most of 
