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 difficulties in the way of con structing 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 con ditions, the greatest distance it has been 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. A device of some importance is the Lightning Arrester. This is used to pro tect the system against lightning dis charges and other abnormally high volt ages. The aluminum arrester is com monly used, which consists of a nest of aluminum cones immersed in an electro lyte, the cones being covered with a film of hydroxide. At a low voltage, only a 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 cur rent falls correspondingly. The arrester acts, so to speak, as a safety valve, re leasing 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 run ning 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 an other important detail in high voltage systems. Oil switches are most common ly used, in which the contacts are im mersed in insulating oil, thus preventing the formation of an arc when a current is broken. They are generally operated at a distance by a system of levers, or by means of a small motor, and impor tant switches are frequently arranged with each pole in a separate chamber of brick or concrete. On the switchboard are assembled ammeters, voltmeters, wattmeters, rheostats, and all the levers for controlling the generating and trans mitting 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 line, caused by fluctuation in consump tion, a storage battery is connected in parallel with the dynamo. When con sumption is small, excess current from the dynamo flows to the batteries and charges them. When consumption is high, current from the dynamo is sup plemented with that from the batteries.