41 6 POPULAR SCIENCE MONTHLY. 



luminous, and sometimes, tlirough accident or otherwise, the cur- 

 rent becomes strong enough to melt the filament, and then the light 

 goes out. In an electric motor it is not necessary to raise the tem- 

 perature of the wire to the melting point to do serious injury; in 

 fact, if the heat is sufficient to char paper or cloth, the machine will 

 be rendered useless until suitable repairs are made. The insula- 

 tion of the wire coils is made principally of cotton, which is a very 

 good electrical insulator in its natural state, but when carbonized 

 by excessive heat it becomes a conductor. As soon as it becomes 

 a conductor the current is no longer confined to the proper channel, 

 but cuts through the insulation to find the shortest path through 

 the machine. If safety fuses were not provided the danger of de- 

 stroying 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 re- 

 duction 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 gener- 

 ators 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 h h h h repre- 

 sent switches by means of which the current is turned on or off 

 from the several tracks. 



