140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
mounted directly on the axle, the field magnets forming a part of 
the locomotive frame, supported by its springs and hence movable 
with regard to the armatures. In this constructoin, therefore, there 
are no armature or field bearings. This locomotive is of the simplest 
type possible, electrically and mechanically, and when operating 
under conditions for which it is properly applicable it has not only 
the highest weight efficiency, but the lowest cost of repairs of any 
direct-current machine, and much lower than is possible for any 
single-phase locomotive. It is structurally a natural high-potential 
machine on account of having but two poles. 
Difference between direct-current and single-phase alternating- 
current motors.—The present inherent differences between direct- 
current and single-phase motors may be briefly summed up as 
follows: 
1. The input of current in one is continuous; in the other inter- 
mittent. 
2. One has a single frame, the electrical and mechanical parts 
being integral; the other has a laminated frame contained within 
an independent casing. Hence there is not equal rigidity, or equal 
use of metal. 
3. One has exposed and hence freely ventilated field coils; the other 
has field coils imbedded in the field magnets. 
4. One has a large polar clearance, and consequently ample bearing 
-wear; the other has an armature clearance of about only one-third 
as much, and hence limited bearing wear. 
5. One is operated with a high magnetic flux, and consequently 
high torque for given armature-conductor current; the other has a 
weak field, and consequent lower armature torque. 
6. One has a moderate sized armature and commutator, and runs 
at a moderate speed; the other, with equal capacity, has a much 
larger diameter of armature and commutator, and runs at a much 
higher speed. 
7. One permits of a low gear reduction, and consequently a large 
gear pitch; the other requires a higher gear reduction and a weaker 
gear pitch. 
8. The windings of one are subject to electrical strains of one 
character; in those of the other the strains are of rapidly variable and 
alternating character. 
9. The mean torque of one is the corresponding maximum; the 
mean torque of the other is only about two-thirds of the maximum. 
10. The torque of one is of continuous character; that of the other 
is variable and pulsating, and changes from nothing to the maximum 
fifty times a second. 
11. One has two or four main poles only, two paths only in the 
armature, and two fixed sets of brushes; the other has four to twelve 
