146 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
brackets carried on the ties, and with the body of the rail about 9 
inches clear (fig. 2). This type of rail has been adopted for the 285 
miles of trackage under electrification, as well as on a number of 
other roads. 
The structure consists, briefly, of a series of iron brackets carried 
on the ties, to the tongued vertical face of which are clamped non- 
charring moisture-proof insulator blocks which loosely embrace the 
head of the rail. Intermediate between the insulators the rail car- 
ries an insulating sheathing, which embraces the head and reaches 
down nearly to the bottom face of the rail, but extends outward from 
the web to form a petticoat protection against snow and sleet. 
For moderate potentials, say of 600 volts, the two halves of the 
insulator blocks are alike, but for the higher potentials the inner in- 
sulator block, that is, the one next to the face of the bracket, is ex- 
tended so as partly to shroud the head of the bracket. The sheath- 
ing between the insulator blocks, depending upon local conditions 
and price of materials, as well as potential used, is formed of three 
wooden strips, one grooved on the under side and inclosing the head 
of the rail, and the other two, attached to and dependent from it, 
reaching in towards the web of the rail. Where good wood is not 
available, an alternate protection, costing about the same and having 
a higher electrical resistance, although not quite so good a mechanical 
one, is a semiflexible shell of indurated fiber conformed to the rail- 
SecilOlUs a 
General comparison of working conductors.—Al\l working conduct- 
ors are in many ways objectionable, but since they are a necessary 
connecting link between the source of supply and the motors, some 
comparisons may be made of the two kinds, the under-contact, pro- 
tected type of third rail and the overhead trolley, as affected by con- 
struction and operation. 
The third rail is an inert structure; it can be aligned accurately 
with the track, is not under strain, and its expansion can be readily 
taken care of. The overhead trolley is necessarily a system under 
strain, and where permanency is desired and high potentials are used 
it must be carried by one or more catenary cables, which on roads of 
high curvature makes the construction more difficult. Its alignment 
in the latter case does not correspond with the line of track, and as 
ordinarily constructed it is subject to- extreme variations of tension 
on account of weather changes. 
The third rail offers some hindrance to the ordinary maintenance 
of track; but overhead construction is inelastic, and the laying of 
additional tracks or changes in grades or alignment require radical 
and expensive alterations or additions in permanent overhead struc- 
tures. 
