Engineering Wonders of the World
Volume III

THE ELECTRIC POWER-STATIONS OF LONDON. 227 system, electricity is generated at a voltage, (or pressure) somewhat higher than that required by the consumers’ lamps, etc., as there is a slight loss in transmission. This method is suited for very short distances only, as the sectional area of each conductor, or main, must be sufficient to carry, without undue resistance, a current or quantity of electricity equal to that used at th© lamps. The above system, which is known as the “ parallel” has been almost altogether superseded by the “ three- wire ” system. In this, electricity is gener- ated by two dynamos joined in “ series ”—i.e., two of their terminals are connected so that the current passes through both machines. This arrangement doubles its pressure but does not alter the quantity. Two The Three. J conductors, which may be wire System. J called the “ outer ” wires, are taken from the remaining terminal of each, machine, and a third from the cable which joins them in series. If the voltage of each, dynamo is, say, 200, the pressure at the “positive” outer conductor will be 200 + 200 = 400 ; that of the middle or third wire, 200 ; and that of the “ negative,” or return, outer wire, 0. There will thus be a potential, or pres- sure, difference of 200 volts between each two neighbouring conductors. From this it will be obvious that 200-volt lamps may be con- nected to the third and to either one of the outer conductors, in spite of the fact that the potential difference of the two outer con- ductors is 400 volts. The importance of this is that the capacity of the two outer con- ductors is, at 400 volts, twice as great as it would be if used in a parallel system at 200 volts, because their sectional area is pro- portional to the quantity, and not to the pressure, of the current they have to transmit. A higher voltage than 250—the present limit of the ordinary incandescent lamp — would be undesirable for domestic use. Electric motors are, however, nearly always “ wound ” for double ths lamp volt- age, and connected to the outer wires of the system. We must now pass on to the alternating system of distribution. The difference be- tween this and the direct system is that, in- stead of a continuous, one- direction current, a series of Alternating , . u , , Current, currents, moving alternately in opposite directions, is set up in the con- ductors. Two complete reversals form a “ period ” or “ cycle,” and the number of these cycles varies from 50 to 100 per second. The most important feature of the alternating current is that the voltage may be raised or lowered, and the current diminished or in- creased in the inverse ratio, by a “ static ” transformer, which, is a simple apparatus con- structed upon the principle of the induction coil, but containing no moving parts. To transform a high-tension direct current to one of low voltage would require a motor suited to the high voltage, and a dynamo designed to give a lower volt- ... Transformers. age with an increase in cur- rent. In other words, it would be necessary to convert electrical energy into mechanical energy (by the motor), and reconvert this me- chanical energy into electrical energy (by the dynamo)—a somewhat inconvenient process compared with the direct method of the static transformer. Owing to the facility with which the voltage of an alternating current can be changed, it is essentially suitable for long- distance transmission, as will be seen later. In some cases the tension adopted is ex- tremely high ; for instance, at the Deptford station of the London Electric Supply Cor- poration the current is generated at 10,500 volts for transmission to various sub-stations, some in the heart of the Metropolis. From the sub-stations the current is distributed, at a reduced tension, by the network of street mains in their immediate neighbourhood. The tension, still high, is further reduced at each house connection by a small transformer in