Transmigration
in comparative religions, metempsychosis; the doctrine of the passage of the soul from one body into another. It appears among many savage races in the form of the belief that ancestral souls return, imparting their own likeness to their descendants and kindred, and Tylor thinks that this notion may have been extended so as to take in the idea of rebirth in bodies of animals. In this form the belief has no ethical value. Transmigration first appears as a factor in the gradual purification of the spiritual part of man, and its return to God, the source and origin of all things, in the religion of the ancient Egyptians, and, according to Herodotus, people of India, whence it passed to the of the characteristic doctrines of Pythagoras, and Pindar the Pythagorean lets the soul return to bliss after passing three unblemished lives on earth. Plato in the dream of Er deals with the condition and treatment of departed souls; and extends the period of the return of souls to God to 10,000 years, during which time they inhabit the bodies of men and animals. Vergil, Persius and Horace allude to it, and Ovid sets forth the philosophy and pre-existences of Pythagoras. Traces of it appear in the "Apocrypha," and that at least some Jews held it in the time of Jesus seems indicated in the disciples' question (John ix: 2). St. Jerome alludes to the existence of a belief in transmigration among the Gnostics, and Origen adopted this belief as the only means of explaining some Scriptural difficulties, such as the struggle of Jacob and Esau before birth (Gen. xxv: 22), and the selection of Jeremiah (Jer. i: 5). In modern times Lessing held it and taught it, and it formed part of the system of Swedenborg. from them to the Greeks. It was one TRANSMISSION OF ELECTRIC POWER. Under this heading are considered methods of conveying electrical energy from one locality to another. The need for such transmission arises chiefly from economic causes. In the center of a city, electricity can seldom be generated except at high cost. It is a common practice to produce the electricity at some point where there is a cheap source of power (such, for instance, as water power, or a readily available and abun- TRANSMISSION dant supply of coal) and then to convey it to the city in the manner outlined below. It is obvious that the site of the central supply station will depend not only on the cost of generating the electricity, but also on the cost of transmitting it to the point where it is to be used. Before considering the methods of transmitting, therefore, it will be advisable to examine, briefly, the different factors determining its cost. The first factor will clearly be the distance which has to be covered. The greater the distance, the more metal will have to be used for conveying the current, and the greater will be the expenditure on constructing and maintaining the service lines. The second factor is the voltage at which the current is supplied.
Electric power is the product of the voltage and the current, and the greater the voltage, the smaller may be the current, and, consequently, the smaller the cross-section of the cable carrying the current. In other words, by using a high voltage and a low current, the amount of copper required will be much smaller than when the current is greater and the voltage power lower, while the amount of power supplies will be no less. Assuming that the efficiency remains constant, the amount of copper required will be proportional to the square of the distance and inversely proportional to the square of the voltage. It is, therefore, economical to have the sources of supply located at a great distance from the point of consumption only when the permissible voltage is very high. It is for this reason that alternating current is always used when supplied over long distances.
Large direct current generators cannot supply current at a higher voltage than 1,500 volts, and so alternating current generators are used. (See DYNAMO- ELECTRIC.) A typical high voltage transmission system consists of the power station in which are located the generators and the "step-up transformers," and the cables to transmit the current to the terminal station, at which are located the "step-down transformers," from which the current is supplied to various sub-stations. The transformer is a piece of apparatus which receives electricity at one voltage and delivers it at another, the "step-up transformers" raising the voltage, while the "step-down transformers" lower it. For many purposes, alternating current is unsuitable or undesirable, and it is frequently necessary to transform the current from alternating to direct. This is done by means of a motor generator set. The motor is driven by the alternating current, and in turn drives a direct current dynamo.
TRANSMISSION The cable for conveying the current is nearly always of copper, but aluminum is occasionally used. The wires are bare and in overhead systems are supported on wooden poles fitted with cross-arms or on steel towers. The former are used for lower voltage systems, the wires being supported on glass or porcelain insulators fixed on to the cross-arms. For higher voltages, however, the steel tower is now commonly used, a suspension type of insulator replacing the so-called pin insulators. Occasionally the cable is laid underground, especially in parts of Europe, where the overhead system is comparatively uncommon. The cable in this case is insulated with paper impregnated with rosin oil, or some similar substance, and is then frequently sheathed in lead.
From what has been said regarding the economy of high voltages, it would seem that the logical procedure would be to generate electricity at the highest possible voltage. There are certain practical considerations, however, which limit the permissible voltage. In the first place, the insulators are not entirely satisfactory at any higher voltage than 60,000. Moreover, when two parallel wires carrying a current are suspended in air, it is found that there is a considerable loss of energy between them, and this loss increases rapidly with voltages above 50,000. The only way to overcome this loss is to keep the wires widely separated, but it is obvious that there is a limit to the possible separation where only one line of poles is used. To obtain satisfactory separation, two or more lines of poles or towers would be necessary, and it will be at once seen that a great increase in cost would result. For this reason, it is found cheaper to limit the voltage, and at the present time 75,000 is the highest voltage that can be considered economical.
Considerable stress is laid on the economical aspect of transmission because this is very often the determining factor in choosing a site for a central station.
One site may be preferable to another as far as amount of available power is concerned, and there may be no insurmountable difficulties in the way of constructing the service lines to the terminal station, but the second site may be chosen on the grounds that cost of transmission will be so much less. To indicate how large a figure this cost reaches in some cases, it may be stated that the cost of transmission equipment for a 150-mile line will amount to at least 20 per cent. and may be as high as 38 per cent. of the total cost of the generating and transmission system. Under present conditions, the greatest distance it has been TRANSMISSION found profitable to transmit electric power is 220 miles. The plants at Niagara now generate upward of 250,000 horse power, and when equipment now in course of construction is complete the figure will be raised to 500,000 horse power. The greatest distance supplied is from Niagara to Syracuse, a distance of 165 miles. Plants in California supply power over a still greater distance, reach ing in one case the high figure of 220 miles.
Some of the details of the transmission equipment may now be briefly described. some importance is the A device of Lightning Arrester. This is used to protect the system against lightning discharges and other abnormally high voltmonly used, which consists of a nest of monly used, which consists of a nest of aluminum cones immersed in an electrolyte, the cones being covered with a film very low current can flow through this contrivance, but when the voltage rises the possible current that it can carry is very high. As soon as the voltage drops below a certain critical point, the high resistance is restored and the current falls correspondingly. The arrester acts, so to speak, as a safety valve, releasing high voltages just as the safety valve on a steam line will release high steam pressure. The actual service line itself is protected from lightning by running a steel wire parallel to the upper wires, this steel wire being grounded at every support. The lightning passes to the ground through this wire, rather than through the copper wires and glass or porcelain insulators. Switches are another important detail in high voltage systems. Oil switches are most commonly used, in which the contacts are imly used, in which the contacts are imthe formation of an arc when a current mersed in insulating oil, thus preventing is broken. They are generally operated at a distance by a system of levers, or by means of a small motor, and important switches are frequently arranged with each pole in a separate chamber of brick or concrete. On the switchboard wattmeters, rheostats, and all the levers for controlling the generating and transmitting system, so that the man in charge can tell almost at a glance the condition of any part of the circuit.
In cases where the area of distribution is small, direct instead of alternating current is used. In order to take care of variation in pressure in the supply current is used. In order to take care tion, a storage battery is connected in parallel with the dynamo. When consumption is small, excess current from the dynamo flows to the batteries and charges them. When consumption is high, current from the dynamo is supplemented with that from the batteries. TRANSPORTATION doing of useful work at a distance from the engine or whatever other source sup-