TRANSMISSION 464 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 elec- tricity, but also on the cost of transmit- ting it to the point where it is to be used. Before considering the methods of transmitting, therefore, it will be advis- ' able to examine, briefly, the different fac-
- tors determining its cost.
The first factor will clearly be the dis- tance which has to be covered. The greater the distance, the more metal will have to be used for conveying the cur- rent, and the greater will be the expendi- ture 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 volt- age and the current, and the greater the voltage, the smaller may be the cur- rent, and, consequently, the smaller the cross-section of the cable carr5dng the current. In other words, by using a high voltage and a low current, the amount of copper required will be much smaller tl:an when the current is greater and the voltage power lower, while the amount of power supplies will be no less. As- suming that the efficiency remains con- stant, the amount of copper required will be proportional to the square of the dis- tance 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 permis- sible 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 trans- mission system consists of the power station in which are located the genera- tors 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 transform- ers" lower it. For many purposes, alter- nating current is unsuitable or undesira- ble, 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. 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 be- ing supported on glass or porcelain in- sulators 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 Eu- rope, where the overhead system is com- paratively 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 pos- sible voltage. There are certain prac- tical 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 con- siderable loss of energy between them, and this loss increases rapidly with volt- ages 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 re- sult. 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 eco- nomical 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 insur- mountable '•'■fficulties in the way of con- structing tho 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 o£ 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 con- ditions, the greatest distance it has been
