41 8 POPULAR SCIENCE MONTHLY. 



upper track; then tlic top generator will feed into the top road 

 only. Tracing the path of the current under these conditions, 

 we find that it will start from the upper side of the generator 

 through the a switch to the B bus, and thence to the trolley wire 

 at the top of the figure. On reaching the first car a portion of the 

 current passes to the track R, the amount being dependent upon 

 the speed of the car and the load. Why the whole current does 

 not follow this path generally puzzles the layman, but the explana- 

 tion is that the motors hold the current back, and only allow as 

 much to pass through them as is necessary to perform the required 

 work — that is to say, the current flowing through each car is not 

 controlled by the generator or by the force of the current, but by 

 the requirements of the motors. The amount of current deliv- 

 ered by the generator is governed by the demands of the motors. 

 The current that does not pass through the first car goes on to 

 the second one, and if there were more cars there would be cur- 

 rent left in the trolley wire to supply them. After passing through 

 the motors of the two cars the current returns through the rails R 

 to the plate D, and thus to bus A, from which it enters the lower 

 side of the top generator. It will from this explanation be seen 

 that the action of the generator is simply to keep the current cir- 

 culating. If two of the generators are connected with the bus 

 bars A and B, the current required by the motors will be delivered 

 by the two machines, and if the three generators are placed in 

 service the current will be divided among them. 



"When two or more generators are used, it is necessary to pro- 

 vide 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 ex- 

 plain the principle upon which this arrangement acts; it is suffi- 

 cient 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. 



