TRANSMISSION OF POWER FOR SUBSTATIONS. 147
taken to carry the wires clear of other circuits, arranging
guard wires for their mutual protection whenever they can
do good. If the circuit runs through a wooded region the
branches of trees and all dead wood should be cleared away
so that nothing can sway against or fall upon the trans-
mission wires. ‘The wire itself should be jointed when nec-
essary with unusual thoroughness, and should be inspected
at the original joints if such there are. It is one advan-
tage of bare wire, that there is no covering to hide careless
joints. 'The line as a whole should be easily reached for
inspection or possible repairs. If it does not run along
the track it should follow a public road or good pathway
so that any point can be quickly reached by wagon or
bicycle.
Up to this point we have been dealing with principles
common to all alternating transmission systems irrespec-
tive of particular characteristics. As a matter of fact or-
dinary single phase alternators are seldom used at present
for transmission purposes. Although the single phase
alternator is, like other dynamos, reversible and can readily
be used as a motor, inability to start as a motor is per-
haps its best known characteristic. This makes it singu-
larly inconvenient for most purposes, and while the diffi-
culty can be overcome by using induction instead of
synchronous motors, single phase induction motors are
not satisfactory for large powers and cause a heavy induct-
ance on the line that is troublesome in more ways than
one. :
For railway distribution it is at present generally nec-
essary to convert the transmitted energy at the substation
into the form of continuous current. The means taken to
do this are quite various, but they all involve the starting
of rotary apparatus, virtually of motors, whatever their
function may be ultimately. So the problem of utilizing
an alternating transmission for railway purposes begins
with the task of starting a synchronous alternating motor.
There have been many ingenious plans devised for this
purpose, some of them depending on the action of a com-
