When two or more generators are used, it is necessary to provide means to prevent the current from dividing unequally between them; if this were not done, one machine might do nearly all the work, while the other one would be practically idle. The means employed to accomplish the result is simply an additional bus bar, which is called an equalizing bus. We will not undertake to explain the principle upon which this arrangement acts; it is sufficient to say that by such means the work can be distributed in amounts directly proportional to the capacity of the generators, so that if one machine is very much larger than the others it will take a portion of the load corresponding to its size. In order that these results may be attained it is necessary to properly adjust the several generators, and as no machine can be made to work with the accuracy of perfection, the work will not be distributed in true proportion for all conditions of load; thus if the generators are adjusted so as to each take its proper share when all the cars are in operation, one machine may do too much or not enough when only one half the number are running, but the excess or deficiency will not be more than a few per cent unless the adjustment is very defective. Electric generators for railway work are made in all sizes, from those only large enough to operate four or five cars to others capable of furnishing sufficient current for thirty or forty or even more. Small generators are made so as to be driven by a belt running over a pulley mounted on the end of the armature shaft, or they may be arranged to be connected to the end of a steam-engine shaft, and thus become what is called direct connected machines. Large generators are almost invariably of the latter type. A machine of this class is illustrated in Fig. 23. The driving engine is shown at _E_, the cylinder being in the background and the crank toward the front, the shaft being clearly seen at _S_, while _F_ is the fly wheel. The generator is mounted directly upon the engine shaft, between the bearing at the crank end and the fly wheel. The large ring marked _G_ is the field magnet ring, and at _D D D_ the field coils are shown. These coils are equally spaced all the way around the circle. The commutator is marked _C_, and the commutator brushes are located at _B B_. The armature can not be seen very well, as it is covered by the brush holders and their supporting frames, but it is located within the ring _G_ in the position designated by _A_. This machine is one of a number used to operate the roads of Troy, N. Y., and is of about one-thousand-horsepower capacity, which is enough to furnish all the current required to run sixty or seventy cars. [Illustration: FIG. 23.--LARGE-SIZE DIRECT CONNECTED ELECTRIC RAILWAY GENERATOR.]