York and Philadelphia and between New York and Chicago. It is not a question of engineering but of finance, and high-speed, heavy electric roads are likely to come when the traffic will support trains at short intervals. The application of electric traction to trains of more than several cars requires a large electric locomotive or a special system of control, so that the motors on each can be controlled by one motorman. The only system in use to-day is the Sprague multiple-unit system, a most ingenious system. (See Street Railway Journal, May, 1901.) The alternating-current motor has been proposed for streetcar work, but it requires two trolley-wires and presents serious problems in speed regulation, so that it is not in use for general railway work. The storage-battery has been proposed, and much money has been spent in trying to use it. The advantages are that each car is a complete and independent unit and that the trolley-lines and feeder-system are abolished; but every line built has failed, and the trolley-system has been substituted. Two disadvantages of the storage-battery for electric traction are the weight of the battery (two tons or more per car) and the rapid deterioration of the battery under street-car conditions. A light, durable storage-battery has been sought but not found. Edison claims to have invented such a battery.
Electric Welding, the process of welding two pieces of metal together by using the heat produced by an electric current. The process developed by Elihu Thomson is practically the only one used. In this the pieces to be welded are held in proper supports, and a large electric current at a very low pressure is sent through the joint from piece to piece. The passage of a large current causes a local production of heat in the metals on each side of the joint, and the metals are pressed together when at the proper temperature, a perfect weld being formed. Practically all kinds of metals can be welded in this way, and even different metals can be welded together, as copper and brass. Welds can also be made by this method in pieces whose shape would prevent welding by ordinary processes. The current used is an alternating current, often of thirty or forty thousand amperes and of correspondingly low pressure. Welding-currents are produced by a special transformer which forms part of the apparatus. The apparatus takes different forms, depending on the shape and size of the work to be handled. For joining two wires it has a very simple form, but for welding pipe or welding artillery-projectiles machines of special form are required.
Electrolysis. Volta, the inventor of the battery, showed that conductors of electricity may be easily divided into two different classes by considering their behavior while an electric current is passing through them. In one group of conductors, as iron, carbon, mercury, etc., we find that the current produces no change in the chemical composition of the body; while in the other group (which includes such bodies as solutions of table-salt, silver-nitrate, copper-sulphate, etc.) the current breaks up the salt, which is so dissolved, that at the point where the current enters the solution we get one constituent of the salt and at the point where the current leaves the solution we get the other constituent of the salt. Bodies which are not decomposed by the current, Volta called conductors of the first class; those which are broken up by the current, he called conductors of the second class. Sir Humphry Davy, in the first decade of the 19th century, established the fact that, when a current is passed through a solution, that solution is, in general, broken up into “two” other substances. It remained for Faraday, however, in 1834, to give an accurate description of the phenomena which accompany the passage of a current through a conductor of the second class. To Faraday, also, we owe the nomenclature of this subject. The wire by which the current enters the solution he called the anode; the wire by which it leaves he called the cathode; the solution itself was called the electrolyte; and the process of decomposition by an electric current was called electrolysis. The parts into which the electrolyte is decomposed are called ions. Faraday's most important discoveries are summarized in the two following laws:
I. “The amount (i. e. the mass) which is decomposed by an electric current is proportional to the current flowing and to the time during which it flows.
II. “When an electrolyte or a series of electrolytes is decomposed by an electric current, the components into which it is separated are always chemically equivalent i. e., they are set free in just such amounts as may recombine and form a chemical compound without any left-over material.”
The applications of electrolysis in commerce are very numerous. Among the most important may be mentioned: (1) plating baser metals with gold, silver or nickel; (2) electrotyping, used in all kinds of printing; (3) the separation of metals, especially copper, from their ores; (4) in defining the legal unit of current, i. e., the ampere; (5) in electro-glazing; (6) in the manufacture and charging of storage-cells. Within the last few years electrolysis has found an unfortunate though important application in the destructive effects of trolley-line currents upon the water-mains in our larger cities. When the return-current leaves the iron pipe and enters the moist ground, a certain amount of the iron is dissolved.
For an excellent brief account of this
