channel, but cuts through the insulation to find the shortest path through the machine. If safety fuses were not provided the danger of destroying the insulation of the motors and thus disabling the car would be decidedly great, for, as already said, the motors can not be stalled with an overload, the only effect produced being a reduction in the speed and an increase in current strength. Now, if there were no way of limiting the increase in current strength the motors, if greatly overloaded, would continue to operate until the insulation gave out. The safety fuse is simply a piece of wire of such size that it will be melted by a current that the motors can carry without being injured; hence when the current strength reaches a point where the safety of the apparatus is endangered the fuse melts and thus breaks the circuit and stops the further flow of current. Fuses are generally made of an alloy that melts at a low temperature, so that the molten metal may not set fire to anything upon which it may fall. These easily fused alloys are inferior to copper as electrical conductors, and on this account the fuse wire is as a rule much larger than that wound upon the motors, which fact makes its action somewhat mysterious to the uninitiated; but whatever its size may be, it is so proportioned that it will melt before the current rises to a strength that would injure the motor coils. The manner in which the electric current generated in the power house reaches the motors is illustrated in Fig. 22. In this figure four tracks are shown, which may be taken to represent roads running in as many different directions. The three squares at the left side represent generators located in the power house. The circles _a a a_ represent switches, by means of which the generators are connected or disconnected from the trolley lines. _A_ and _B_ represent heavy metallic rods, generally made of copper, with which the generators are connected by means of the switches _a a a_. These rods are called bus bars. The circles _b b b b_ represent switches by means of which the current is turned on or off from the several tracks. [Illustration: FIG. 22.--DIAGRAM ILLUSTRATING THE MANNER IN WHICH THE ELECTRIC CURRENT FLOWS FROM THE GENERATORS TO THE CARS UPON THE TRACKS.]