poles or the underground trolley in the conduit, or the third rail laid at the side of the track.  All of these have become standard practice and are operated at the usual voltage of from 500 to 600 volts.  The current on lines of any considerable length is alternating current, supplied from large central generating stations and transformed to direct as occasion may demand at suitable sub-stations.  Recently there has been a tendency to employ high voltage direct current systems where the advantages of the use of direct current motors are combined with the economies of high voltage transmission, chief of which are the avoiding of power losses in transmission and the economy in the first cost of copper.  These high voltage direct current lines were first used in Europe, and during the year 1907 experimental lines on the Vienna railway were tested.  In Germany and Switzerland tests were made of direct current system of 2,000 and 3,000 volts and in 1908 there was completed the first section of a 1,200-volt direct current line between Indianapolis and Louisville, which marked the first use of high tension direct current in the United States, and this was followed by other successful installations.

With alternating current there can be used the various forms of single phase or polyphase current familiar in power work, but the latter is now preferred, and in Europe and in the United States in the latter part of 1908 the number of single phase lines was estimated at 27 and 28 respectively, with a total mileage of 782 and 967 miles.  A trolley wire or suspended conductor is used.  To employ a single phase current, motors of either the repulsion type or of the series type are used and are of heavier weight than the direct current motors, as they must combine the functions of a transformer and a motor.  It is for this reason that we often see two electric locomotives at the head of a single train on lines where the single phase system is employed, while on neighboring lines using direct current, one locomotive of hardly larger size suffices.  With the polyphase current a motor with a rotating field is used, and they have considerable efficiency as regards weight when compared with the single phase and with the direct current motor.  The polyphase motor, however, is open to the objection that it does not lend itself to regulations as well as the direct current form, and with ingenious devices involving the arrangement of the magnetic field and the combination of motors, various running speeds can be had.  The usual voltage for these motors is 3,000 volts, but in the polyphase plant designed for the Cascade Tunnel 6,000 volts are to be used.  They possess many advantages, especially their ability to run at overload, and consequently a locomotive with polyphase motor will run up grade without serious loss of speed.  The single phase system has been carried on on Swiss and Italian railroads, notably on the Simplon Tunnel and the Baltelina lines with