ELECTRIC TRUNK-LINE OPERATION—SPRAGUE. 141 
poles, as many paths in the armature, leading to unbalancing, and 
as many movable sets of commutator brushes. 
12. One can maintain a high torque for a considerable time while 
standing still; the other is apt to burn out the coils which are short 
circuited under the brushes. 
18. In one, all armature-coil connections are made directly to the 
commutator; in the other, on the larger sizes, resistances are intro- 
duced between the coils and every bar of the commutator, some of 
which are always in circuit, and the remainder always present. 
14. In one the sustained capacity for a given weight is within the 
reasonable requirements of construction; in the other it is only about 
half as much. 
15. Finally, the gearless type, with armature and field varying 
relatively to each other, is available for one, but this construction is 
denied to the other.. 
Consideration, then, of the characteristics peculiar to each class of 
motor indicate, not that the single-phase motor can not be used, but 
that if adopted the weight or number, and the cost of locomotives or 
motors required to do the work must be much greater; that the depre- 
ciation of that which is in motion will be higher; and that there 
will always be an excess weight of fixed amount per unit which must 
be carried irrespective of the trailing or effective loads. We must, 
therefore, in many cases be led to the selection of the direct-current 
motor, that motor which has the higher weight capacity, the greater 
endurance, and the lower cost per unit of power. 
Electric braking.—Recuperation of energy to reduce the amount 
of power used, and to make the motors act as brakes to retard the 
acceleration of a train, has been a favorite project, and attended 
by many prophecies since the beginning of the electric railway 
imGustiya) #s =. * 
On ordinary railroads the gradients are not sufficient to make it 
worth while to attempt any recuperation of energy; the acceleration 
due to any excess of gravity coefficient above that necessary to over- 
come the friction of the train is usually welcomed. On mountain 
roads, however, electric braking may become an important adjunct, 
not because of power economy, but for safer operation. In any case, 
simplicity of application and absolute reliability of action are first 
essentials. 
There are two general methods available: One in which the energy 
of the descending train drives the motors, acting with shunt or inde- 
pendent field characteristics, at an aggregate potential above that of 
the line at the rail, and sending current back into the line; another in 
which the motors are disconnected from the line, and driven as self- 
exciting generators on a closed circuit, as much of the energy of the 
descending train as desired being used up in heating rheostats. * * * 
