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 great success, and the distribution problems are reduced to a minimum. In the United States a notable installation has been on the New York, New Haven & Hartford Railroad, where the section between Stamford and New York has been worked by electricity exclusively since July 1, 1908. Here the single phase motors use direct current while running over the tracks of the New York Central from Woodlawn to the Grand Central Terminal. On both the New York, New Haven & Hartford and the New York Central locomotives the armature is formed directly on the axle of the driving wheels, so consequently much interest attaches to the new design adopted for the Pennsylvania tunnels, where the armatures of the direct current motors are connected with the driving wheels by connecting rods somewhat after the fashion of the steam locomotive, and following in this respect some successful European practice. APPENDIX. UNITS OF MEASUREMENT. (From Munro and Jamieson's Pocket-book of Electrical Rules and Tables). I. FUNDAMENTAL UNITS.--The electrical units are derived from the following mechanical units:-- The Centimetre as a unit of length; The Gramme as a unit of mass; The Second as a unit of time.