One of the most interesting features of electric-power transmission is the care that is taken to avoid the terrible danger from the high potentials, and at the same time prevent loss of power on the way. The electricity in the machinery and in the line-wires that extend across the country is veritable lightning, and has to be carefully guarded from doing any damage or escaping. To prevent leakage, the insulation of all of the station machinery and apparatus is made extra good, with “high dielectric strength,” so that it will not be punctured by the high voltage; and the line-insulators are made very large, and electrically and mechanically strong--quite unlike the ordinary-sized glass or porcelain insulators that are employed for telegraph and telephone lines. Each insulator before being put up is tested under a “breakdown voltage” much higher than it is to stand in actual service. Oil-switches The switching of high-voltage electric power is a knotty problem. The circuit cannot be interrupted by “air-break” switches, such as are used in ordinary electric-light stations, for any attempt to do so would result in a destructive arc many feet long, that could not be extinguished. Therefore “oil-switches” are always used to control the line-circuits at the main generating station and the sub-stations. In these oil-switches--which are designed to be operated from a distance, by hand-levers, or sometimes by electric motors--the circuit is made and broken under the surface of oil, which prevents the formation of an arc. Moreover, the switchboard attendant does not have to come anywhere near the deadly high-voltage wires, but can make the necessary connections at a safe distance. Electric Traction The use of the electric motor to propel vehicles of all kinds is called electric traction. It is, of course, a branch of electric power, which we have just been considering; and it is in many respects the most important branch. The wealth of a country is largely built up and maintained by its facilities for transportation, such as its canals, highways, railroads, and street and interurban car-lines. In this field electric power is playing a most important part, although it was not many years ago that the first experimental electric cars were put in to replace horses on the street-railways of our cities. The change was found to be so successful that the field of the trolley-car was widened and extended very rapidly, until now we have our great suburban and interurban electric railways, with cars almost or quite as big as those on the steam-railroads and running at even higher speeds. During the last few years, also, the sphere of the steam-railroad itself has been invaded by electricity, by the construction of powerful electric locomotives to draw passenger and freight trains. The Trolley-car