138 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
ning at a trifle of over 2 to 1, rises to more than 4 to 1 in favor of 
the direct-current motor on the longer runs. This comparison of 
weight coefficients does not include the collectors, control switches, 
rheostats, transformers, or wiring, which in the aggregate are 
enough heavier for the alternating-current motor to maintain these 
disparities. 
It is evident, therefore, that a pair of these alternating-current 
motors can handle only about one-half of the total load of the direct- 
current motors, with all the disadvantage of higher armature speed 
and smaller air gaps; and considering the excess weight of the con- 
trol apparatus, the net load over and above the electric equipment 
would be considerably less than one-half. 
This general comparison is not, so far as the relative characteristics 
are concerned, individual to this particular size of motor, but seems 
at present to be equally applicable through a wide range, and indif- 
ferently as to the make, or whether the alternating-current motor is 
of the series-compensated or the repulsion type. 
Polyphase and direct-current motor characteristics—Opposed to 
the two types of single-phase motors are the polyphase and the direct- 
current motors, the former with a rotating field and the latter with 
a field of fixed character. They have similarly high weight effi- 
ciencies, the former having somewhat the advantage when compared 
with the ordinary type of direct-current motor. 
The polyphase motor, however, is a normally constant-speed ma- 
chine. It can, through a rearrangement of fields, be run at two 
different speeds, but each is practically a constant one. Or where 
there is a plurality of motors, half and full speed can be obtained by 
having one pair of different character from the other, and operated 
in cascade relation to it, with the necessity, however, of throwing one 
pair of motors out of service when running at full speed. With 
rascade operation and field changing combined there can be three run- 
ning speeds. These motors, so far as their supply is concerned, have 
been limited for practical reasons to a potential of 3,000 volts on the 
trolley, because the supply requires, besides the rail, two wires over- 
head, although a recent undertaking in the United States, the opera- 
tion in the Cascade Tunnel, is to be attempted at 6,000 volts. 
Polyphase motors have an enormous overload capacity, and the fact 
that they run at synchronous speeds with a very small slip, and if the 
frequency is unchanged will run upgrade nearly as fast as on a level, 
would indicate at first sight excessive loads on main and substations. 
But a curious and perfectly natural result has been pointed out by 
Cserhati and Von Kando, namely, that with suitable provision for 
regulation at the central station, so that with excessive loads the gen- 
erators will drop in speed, there will, with such speed reduction, be 
