ELECTRIC TRUNK-LINE OPERATION—-SPRAGUE. 159 
With an alternating-current system, the battery at a substation 
plays an entirely different role. It must be charged by a direct-cur- 
rent generator driven by an alternating-current motor, and in dis- 
charging drives the alternating-current motor as a dynamo through 
the direct-current generator acting as a motor. In addition to the 
introduction of moving machinery in an alternating-current sub- 
station, its watt capacity must be equal to that of the full discharge 
of the battery, and the latter can have no function in supplying 
current to the working conductor except through the medium of two 
rotating machines of large capacity. 
Use of step-up and step-down transformers.—Where the distance 
is not great, as on the present proposed limited operation of the 
New Haven road, both step-up and step-down transformers have 
been omitted, and the 11,000-volt trolley line is supplied from the 
station switchboard. This means direct connection between an ex- 
tended system of overhead working conductors and generators op- 
erated at high potential, with one side grounded, with, of course, 
whatever protection lightning arresters can provide. Such are the 
vagaries of lightning and the uncertainty of the very best arresters, 
that I cannot but feel that this practice, which subjects costly gen- 
erating equipments to direct lightning attack and special grounding 
stress, will not obtain to any great extent; for the possibility of lay- 
ing up a complete unit of great capacity, steam engine as well as 
generator, because of a lightning flash or accidental ground, is too 
great a penalty to pay for eliminating transformers, and is a special 
handicap upon the possibilities of transmission. 
It is certain that standardization should be directed to the con- 
struction of generators. Any material increase of potential above 
that now common means reduced capacity and efficiency, increased 
danger of breakdown, and greatly increases individual cost, to say 
nothing of the capitalized risk of failure. Quite aside from the 
question of cost and efficiency, air cooling, the only possible method 
for generators, manifestly cannot be safely carried above that which 
is tolerable for static transformers, which, when wound for the higher 
potentials, are invariably oil-cooled. Therefore I expect to see stand- 
ardization of generator potentials, the pressure being stepped up by 
transformers to whatever transmission potential is necessary, and 
then stepped down to the working pressure on the trolley wire if 
alternating current be used, or to a lower pressure and converted if 
direct current be used. 
The transformer, per se, is the simplest and most flexible device for 
changing alternating-current volume and pressure, and its moderate 
cost and high efficiency, taken in connection with the like elements of 
moderately high potential generators, will leave the total cost and 
efliciency of generating equipment roughly the same. There will be 
