雅思阅读第099套P2-AC or DC: The War of Currents

AC or DC: The War of Currents

Electricity can be delivered in either alternating current (AC), or direct current (DC), and in the late 1880s in America, with electricity delivery in its infancy, it initially seemed clear which system was superior. Thomas Edison, a home-grown American inventor, heavily favoured DC from the start. Yet the limitations of his system would become increasingly obvious, as would the advantages of AC, and despite Edison’s best efforts, his crusade would ultimately be lost.

In 1879, Edison’s team at Menlo Park had improved the electric light bulb, but Edison needed an efficient electricity distribution system to capitalise on this. Thus, in 1880, he founded the Edison Illuminating Company and constructed a generating station providing 110 volts of direct current. Yet such a system has drawbacks. Due to the low voltage, there Hows correspondingly higher current, meaning that the electrical resistance of the transmission wires significantly reduces the voltage as it travels further afield. Whatever thickness of wire is used, there is a natural limitation in the distance over which the electricity can be economically transmitted.

There are, however, benefits to using DC. It can allow storage batteries to be directly connected to the electricity grid, giving extra power to meet sudden short-term peaks of demand, or backup during breakdown of supply. Furthermore, during Edison’s time, there were no practical AC motors, only DC ones. Also, most of the load consisted of incandescent light bulbs, which ran well on DC. Perhaps most importantly, Edison had the patents (legal rights) to many associated DC devices which he and his team had invented, such as meters, telegraphic devices, and household machinery. Thus, the widespread adoption of DC across America would see him gain considerably from patent royalties.

Still, all such inventions were somewhat useless when DC electricity could only be delivered to customers within a few kilometers of the generating source. To overcome this problem, the best answer is to transform, or step-up, the voltage to very high levels for transmission, and then transform it down to safe levels for customer use. This also allows thinner and less expensive wires, but there is no low-cost technology to transform voltage — unless one uses AC, and it was the brilliant physicist and prolific inventor, Nikola Tesla, who had extensively researched this system.

Tesla, a penniless immigrant from Serbia, worked for a year at Edison’s Menlo lab. He had actually proposed the AC system to Edison, but Edison, an empirical experimenter with little formal education, dismissed it as impractical. Tesla, with the mathematical training and formal theoretical knowledge, was able to understand AC’s potential, even inventing an AC polyphase electric induction motor. Tesla soon felt he was not being given due credit or enough financial compensation from Edison, and a direct confrontation led to him immediately resigning, after which he was reduced to working as a labourer for a few years to make ends meet.

But Tesla was not the first to advocate AC. The system was being atrialed in many European countries, with considerable success. One of the converts to the cause was a university-trained electrical engineer named George Westinghouse, and he was willing to invest in the idea. He formed a company and purchased the patents to AC-based transformer technology from its European inventors, as well those to Tesla’s AC polyphase electric motor, among others. This eventually led to him hiring Tesla himself to help commercialise AC, and promote it as a better system. A bitter feud, known as the ‘War of Currents’ was set to begin.

Edison first strike was to claim that high-voltage systems were too dangerous to use. Certainly they were dangerous, but Westinghouse countered that such risks could be minimised and were considerably outweighed by the benefits. Edison’s next strike was to use his influence on various American state governments to limit power transmission to low voltages, effectively eliminating AC from the competition. When this failed, Edison was prepared to conduct public electrocutions of animals by AC — even on a rogue elephant no longer wanted by its circus owners. In the battle of public opinion, this was even filmed.

The next logical step was to show AC’s deadliness on human beings themselves. Edison, realising that he was losing the war, again used his influence on government, this time to promote the use of AC for the execution of prisoners. Thus, in 1890, the first ‘electric chair’ was constructed in anticipation of an impending death sentence. Westinghouse countered by hiring the best lawyers of the day to defend the prisoner in question, as well as to prevent the system of execution. Although he failed in both respects, the results were unexpected. Despite a botched execution and the horror of the spectators, the electric chair would remain, but AC would not be stigmatised as the killer Edison’s had hoped.

Meanwhile, AC’s range and efficiency saw Westinghouse being given high prestige engineering projects, such as the Ames Hydro-electric Generating Plant (1891), and another one on Niagara Falls, culminating in the greatest public relations victory in 1893: the contract to illuminate the Columbian Exposition in Chicago. Here, Tesla and Westinghouse showed the wonders of AC power with various electrical exhibits, such as fluorescent lamps and Tesla’s AC motors, to an awestruck audience and widespread press attention. After that, the war was effectively won, and AC would take over almost completely.

SECTION 2: QUESTIONS 14-26

Questions 14-17

Complete the table.

Choose NO MORE THAN TWO WORDS from the passage for each answer.

Direct Current

Advantages	Disadvantages
Supply can be supplemented with 14 _________________	Wire resistance lowers the 15 _________________
could power 16 _________________ motors (only sort available then)	Supply distance is limited.
gave Edison income	Wires used are thick and 17 _________________