All the principles of generating electricity had been worked out in the 19th Century, but by its end these had only just begun to produce electricity on a large scale. The 20th Century has witnessed a colossal expansion of electrical power generation and distribution. The general pattern has been toward ever-larger units of production, using steam from coal- or oil-fired boilers. Economies of scale and the greater physical efficiency achieved as higher steam temperatures and pressures were attained both reinforced this tendency. U.S. experience indicates the trend: in the first decade of the century a generating unit with a capacity of 25,000 kilowatts with pressures up to 200-300 pounds per square inch at 400º-500º F (about 200º-265º C) was considered large, but by 1930 the largest unit was 208,000 kilowatts, with pressures of 1,200 pounds per square inch at a temperature of 725º F, while the amount of fuel necessary to produce a kilowatt-hour of electricity and the price to the consumer had fallen dramatically.
As the market for electricity increased, so did the distance over which it was transmitted, and the efficiency of transmission required higher and higher voltages. The small direct-current generators of early urban power systems were abandoned in favor of alternating-current systems, which could be adapted more readily to high voltages. Transmission over a line of 155 miles (250 kilometers) was established in California in 1908 at 110,000 volts; Hoover Dam in the 1930s used a line of 300 miles (480 kilometers) at 287,000 volts. The latter case may serve as a reminder that hydroelectric power, using a fall of water to drive water turbines, has been developed to generate electricity where the climate and topography make it possible to combine production with convenient transmission to a market. Remarkable levels of efficiency have been achieved in modern plants.
One important consequence of the ever-expanding consumption of electricity in the industrialized countries has been the linking of local systems to provide vast power grids, or pools, within which power can be shifted easily to meet changing local needs for current.
AC has other advantages:
- AC generators are simple, cheaper and more reliable than DC generators
- AC can readily be switched by circuit breakers at any voltage, whereas DC can only be switched at low voltages
- AC motors and other electrical appliances are cheaper, simpler, and more reliable than those designed to work with DC
- The frequency can be very precisely controlled and so AC is useful in motors that require accurate speed eg. Clocks, tape recorders, VHS machines.
So, while Thomas Edison receives the greater part of credit, it is clear that we owe respect and gratitude for Nikola Tesla’s creative and intelligent mind.
At a local radio shop Tesla bought 12 vacuum tubes, some wires and assorted resistors, and assembled them in a circuit box 24 inches long, 12 inches wide and 6 inches high, with a pair of 3-inch rods sticking out. Getting into the car with the circuit box in the front seat beside him, he pushed the rods in, announced, “We now have power,” and proceeded to test drive the car for a week, often at speeds of up to 90 mph. As it was an alternating-current motor and there were no batteries involved, where did the power come from?