“Transatlantic telegraph cable”的意思、由来-开放百科全书

A transatlantic telegraph cable is an undersea cable running under the Atlantic Ocean used for telegraph communications. The first was laid across the floor of the Atlantic from Telegraph Field, Foilhommerum Bay, Valentia Island in western Ireland to Heart's Content in eastern Newfoundland. The first communications occurred August 16, 1858, reducing the communication time between North America and Europe from ten days – the time it took to deliver a message by ship – to a matter of minutes. Transatlantic telegraph cables have been replaced by transatlantic telecommunications cables.

Early history

In the 1840s and 1850s several individuals proposed or advocated construction of a telegraph cable across the Atlantic Ocean, including Edward Thornton and Alonzo Jackman.^[1] Cyrus West Field and the Atlantic Telegraph Company were behind the construction of the first transatlantic telegraph cable.^[2] The project began in 1854 and was completed in 1858. The cable functioned for only three weeks, but it was the first such project to yield practical results. The first official telegram to pass between two continents was a letter of congratulations from Queen Victoria of the United Kingdom to then President of the United States, James Buchanan on August 16. Signal quality declined rapidly, slowing transmission to an almost unusable speed. The cable was destroyed the following month when Wildman Whitehouse applied excessive voltage to it while trying to achieve faster operation. It has been argued that the faulty manufacture, storage and handling of the 1858 cable would have led to premature failure in any case.^[3] The cable's rapid failure undermined public and investor confidence and delayed efforts to restore a connection. A second attempt was undertaken in 1865 with much-improved material and, following some setbacks, a connection was completed and put into service on July 28, 1866. This cable proved more durable.

Before the first transatlantic cable, communications between Europe and the Americas took place only by ship. Sometimes, however, severe winter storms delayed ships for weeks. The transatlantic cable reduced communication time considerably, allowing a message and a response in the same day. Five attempts to lay a cable were made over a nine-year period – one in 1857, two in 1858, one in 1865, and one in 1866. Lasting connections were finally achieved with the 1866 cable and the 1865 cable, which was repaired by Isambard Kingdom Brunel's ship {{SS|Great Eastern||6}}, captained by Sir James Anderson.^[4] In the 1870s duplex and quadruplex transmission and receiving systems were set up that could relay multiple messages over the cable.{{Citation needed|date=October 2011|reason=In the article Electrical telegraph, there is said at the end of the section Transatlantic era that Edison got a patent on what was then called the 'Duplex Telegraph' only in 1892.}}

Additional cables were laid between Foilhommerum and Heart's Content in 1873, 1874, 1880, and 1894. By the end of the 19th century, British-, French-, German-, and American-owned cables linked Europe and North America in a sophisticated web of telegraphic communications.{{Citation needed|date=August 2016|reason=Need a citation for both sentences here.}}

Origins of the idea

William Cooke and Charles Wheatstone introduced their working telegraph in 1839. As early as 1840 Samuel F. B. Morse proclaimed his faith in the idea of a submarine line across the Atlantic Ocean. By 1850 a cable was run between England and France. That same year Bishop John T. Mullock, head of the Roman Catholic Church in Newfoundland, proposed a telegraph line through the forest from St. John's to Cape Ray, and cables across the Gulf of St. Lawrence from Cape Ray to Nova Scotia across the Cabot Strait.

At about the same time a similar plan occurred to Frederick Newton Gisborne, a telegraph engineer in Nova Scotia. In the spring of 1851, Gisborne procured a grant from the legislature of Newfoundland and, having formed a company, began the construction of the landline. In 1853 his company collapsed, he was arrested for debt and he lost everything. The following year he was introduced to Cyrus West Field. Field invited Gisborne to his house to discuss the project. From his visitor, Field considered the idea that the cable to Newfoundland might be extended across the Atlantic Ocean.

Field was ignorant of submarine cables and the deep sea. He consulted Morse as well as Lieutenant Matthew Maury, an authority on oceanography. Field adopted Gisborne's scheme as a preliminary step to the bigger undertaking, and promoted the New York, Newfoundland and London Telegraph Company to establish a telegraph line between America and Europe.

St. John's to Nova Scotia

The first step was to finish the line between St. John's and Nova Scotia, and in 1855 an attempt was made to lay a cable across the Cabot Strait in the Gulf of Saint Lawrence. It was laid out from a barque in tow of a steamer. When half the cable was laid, a gale rose, and the line was cut to keep the barque from sinking. In 1856 a steamboat was fitted out for the purpose, and the link from Cape Ray, Newfoundland to Aspy Bay, Nova Scotia was successfully laid.^[5]

Transatlantic

With Charles Tilston Bright as chief engineer, Field then directed the transoceanic cable effort. A survey was made of the proposed route and showed that the cable was feasible. Funds were raised from both American and British sources by selling shares in the Atlantic Telegraph Company. Field himself supplied a quarter of the needed capital.

The cable consisted of seven copper wires, each weighing 26 kg/km (107 pounds per nautical mile), covered with three coats of gutta-percha (as suggested by Jonathan Nash Hearder^[6]), weighing 64 kg/km (261 pounds per nautical mile), and wound with tarred hemp, over which a sheath of 18 strands, each of seven iron wires, was laid in a close helix. It weighed nearly 550 kg/km (1.1 tons per nautical mile), was relatively flexible and was able to withstand a pull of several tens of kilonewtons (several tons). It was made jointly by two English firms – Glass, Elliot & Co., of Greenwich, and R.S. Newall and Company, of Birkenhead. Late in manufacturing it was discovered that the respective sections had been made with strands twisted in opposite directions. While the two sections proved a simple matter to join, this mistake subsequently became magnified in the public mind.^[7]

The British government gave Field a subsidy of £1,400 a year (£{{inflation|UK|1400|1856|fmt=c|r=-4}} today) and loaned the ships needed. Field also solicited aid from the U.S. government. A bill authorizing a subsidy was submitted in Congress. The subsidy bill passed the Senate by a single vote, due to opposition from protectionist senators. In the House of Representatives, the bill encountered similar resistance, but passed, and was signed by President Franklin Pierce.

The first attempt, in 1857, was a failure. The cable-laying vessels were the converted warships HMS Agamemnon and USS Niagara. The cable was started at the white strand near Ballycarbery Castle in County Kerry, on the southwest coast of Ireland, on August 5, 1857.^[8] The cable broke on the first day, but was grappled and repaired; it broke again over the "telegraph plateau", nearly 3,200 m (2 statute miles) deep, and the operation was abandoned for the year.

The following summer, after experiments in the Bay of Biscay, Agamemnon and Niagara tried again. The vessels were to meet in the middle of the Atlantic, where the two halves of the cable were to be spliced together, and while Agamemnon paid out eastwards to Valentia Island, Niagara was to pay out westward to Newfoundland. On June 26, the middle splice was made and the cable was dropped. Again the cable broke, the first time after less than 5.5 km (three nautical miles), again after some 100 km (54 nautical miles) and for a third time when about 370 km (200 nautical miles) of cable had run out of each vessel.

The expedition returned to Queenstown, and set out again on July 17, with little enthusiasm among the crews. The middle splice was finished on July 29, 1858. The cable ran easily this time. Niagara arrived in Trinity Bay, Newfoundland on August 4 and the next morning the shore end was landed. Agamemnon made an equally successful run. On August 5, she arrived at Valentia Island, and the shore end was landed at Knightstown and then laid to the nearby cable house.^[9]

First contact

Test messages were sent from Newfoundland beginning August 10, 1858; the first successfully read at Valentia was on August 12, and the first successfully read at Newfoundland on August 13; further test and configuration messages followed until August 16, when the first official message was sent via the cable: "Directors of Atlantic Telegraph Company, Great Britain, to Directors in America:—Europe and America are united by telegraph. Glory to God in the highest; on earth peace, good will towards men."^[12]^[13]^[14] Next was the text of a telegram of congratulation from Queen Victoria to President James Buchanan at his summer residence in the Bedford Springs Hotel in Pennsylvania and expressed a hope that it would prove "an additional link between the nations whose friendship is founded on their common interest and reciprocal esteem." The President responded that, "it is a triumph more glorious, because far more useful to mankind, than was ever won by conqueror on the field of battle. May the Atlantic telegraph, under the blessing of Heaven, prove to be a bond of perpetual peace and friendship between the kindred nations, and an instrument destined by Divine Providence to diffuse religion, civilization, liberty, and law throughout the world."^[15] The messages were hard to decipher – Queen Victoria's message of 98 words took sixteen hours to send.^[12]^[16]

These messages engendered an outburst of enthusiasm. The next morning a grand salute of 100 guns resounded in New York City, the streets were decorated with flags, the bells of the churches were rung, and at night the city was illuminated.^[17]. On September 1 there was a parade, followed by a evening torchlight procession and fireworks display, which caused a fire in the Town Hall.^[18] Bright was knighted for his part, the first such honour to the telegraph industry.^[19]

Failure of the first cable

The operation of the new cable was plagued by the fact that the two senior electrical engineers of the company had very different ideas on how the cable should be worked.^[20] Lord Kelvin and Dr Wildman Whitehouse were located at opposite ends of the cable, communicating only by the cable itself.^[2]

Kelvin, located at the western end, believed that it was necessary to employ only a low voltage and to detect the rising edge of the current flowing out of the cable and, once this had been done, nothing would be gained by further monitoring (Morse code used a positive current for a 'dot' and a negative current for a 'dash'). Kelvin invented his mirror galvanometer precisely for this task of observing the current change quickly.

At the eastern end of the cable was Whitehouse. He was the company's chief electrician and a doctor of medicine – any electrical knowledge that he possessed was self-taught. Whitehouse believed that, in order to have the current at the receiving end change as rapidly as possible, the cable should be driven from a high-voltage source (several thousand volts from induction coils). The position was made worse because every time intelligible Morse code was seen on the mirror galvanometer at the eastern end, Whitehouse insisted that the galvanometer be disconnected and replaced with his own patented telegraph recorder, which was far less sensitive.

The effects of the poor handling and design of the cable, coupled with Whitehouse's repeated attempts to drive the cable with high voltages, resulted in the insulation of the cable being compromised. All the while, it was taking longer and longer to send messages. Towards the end, sending half a page of message text was taking as long as a day.

In September 1858, after several days of progressive deterioration of the insulation, the cable failed. The reaction to this news was tremendous. Some writers even hinted that the line was a mere hoax, and others pronounced it a stock exchange speculation. In the enquiry that followed, Dr Whitehouse was deemed responsible for the failure, and the company did not escape criticism for employing an electrical engineer with no recognised qualifications.

Although the cable was never put into service for public use, and never worked well, there was time for a few messages to be passed that went beyond testing. The collision between the Cunard Line ships Europa and Arabia was reported on 17 August. The British Government used the cable to countermand an order for two regiments in Canada to embark for England, saving £50,000. A total of 732 messages were passed before failure.^[21]

Field was undaunted by the failure. He was eager to renew the work, but the public had lost confidence in the scheme and his efforts to revive the company were futile. It was not until 1864 that, with the assistance of Thomas Brassey and John Pender, he succeeded in raising the necessary capital. The Glass, Elliot, and Gutta-Percha Companies were united to form the Telegraph Construction and Maintenance Company (Telcon, later part of BICC), which undertook to manufacture and lay the new cable. C.F. Varley replaced Whitehouse as chief electrician.^[2]

In the meantime, long cables had been submerged in the Mediterranean and the Red Sea. With this experience, an improved cable was designed. The core consisted of seven twisted strands of very pure copper weighing 300 pounds per nautical mile (73 kg/km), coated with Chatterton's compound, then covered with four layers of gutta-percha, alternating with four thin layers of the compound cementing the whole, and bringing the weight of the insulator to 400 lb/nmi (98 kg/km). This core was covered with hemp saturated in a preservative solution, and on the hemp were helically wound eighteen single strands of high tensile steel wire produced by Webster & Horsfall Ltd of Hay Mills Birmingham, each covered with fine strands of manila yarn steeped in the preservative. The weight of the new cable was 35.75 long hundredweight (4000 lb) per nautical mile (980 kg/km), or nearly twice the weight of the old. The Haymills site successfully manufactured {{convert|30,000|mi|km}} of wire (1,600 tons), made by 250 workers over eleven months.

Repairing the cable

Broken cables required an elaborate repair procedure. The approximate distance to the break is determined by measuring the resistance of the broken cable. The repair ship navigated to the location. The cable was hooked with a grapple and brought on board to test for electrical continuity. Buoys were deployed to mark the ends of good cable and a splice was made between the two ends.^[22]^[23]

Great Eastern

The new cable was laid by the ship Great Eastern captained by Sir James Anderson.^[24] Her immense hull was fitted with three iron tanks for the reception of {{convert|2,300|nmi|km}} of cable, and her decks furnished with the paying-out gear. At noon on July 15, 1865, Great Eastern left the Nore for Foilhommerum Bay, Valentia Island, where the shore end was laid by Caroline. This attempt failed on July 31 when, after 1,062 miles (1968 km) had been paid out, the cable snapped near the stern of the ship, and the end was lost.^[25]

Great Eastern steamed back to England, where Field issued another prospectus, and formed the Anglo-American Telegraph Company,^[26] to lay a new cable and complete the broken one. On July 13, 1866, Great Eastern started paying out once more. Despite problems with the weather on the evening of Friday, July 27, the expedition reached the port of Heart's Content in a thick fog. The next morning at 9 a.m. a message from England cited these words from the leader in The Times: "It is a great work, a glory to our age and nation, and the men who have achieved it deserve to be honoured among the benefactors of their race." "Treaty of peace signed between Prussia and Austria." The shore end was landed during the day by Medway. Congratulations poured in, and friendly telegrams were again exchanged between Queen Victoria and the United States.

On August 9 Great Eastern put to sea again, in order to grapple the lost cable of 1865, and complete it to Newfoundland.^[27] They were determined to find it. There were some who thought it hopeless to try, declaring that to locate a cable {{cvt|2.5|mi|||}} down would be like looking for a small needle in a large haystack. Robert Halpin navigated the ship to the correct location.^[28] For days, Great Eastern moved slowly here and there, "fishing" for the lost cable with a grappling hook at the end of a stout rope. Suddenly, the cable was "caught" and brought to the surface, but while the men cheered it slipped from the hook and vanished again. It was not until a fortnight later that it was once more fished up; it took 26 hours to get it safely on board Great Eastern again. The cable was carried to the electrician's room, where it was determined that the cable was connected. All on the ship cheered or wept as rockets were sent up into the sky to light the sea. The recovered cable was then spliced to a fresh cable in her hold, and paid out to Heart's Content, Newfoundland, where she arrived on Saturday, September 7. There were now two working telegraph lines.

Communication speeds

Initially messages were sent by an operator sending Morse code. The reception was very bad on the 1858 cable, and it took two minutes to transmit just one character (a single letter or a single number), a rate of about 0.1 words per minute. This is despite the use of a highly sensitive mirror galvanometer, a new invention at the time.

The first message on the 1858 cable took over 17 hours to transmit.^[29] For the 1866 cable, the methods of cable manufacture, as well as sending messages, had been vastly improved. The 1866 cable could transmit eight words a minute^[30] – 80 times faster than the 1858 cable. Heaviside and Mihajlo Idvorski Pupin in later decades understood that the bandwidth of a cable is hindered by an imbalance between capacitive and inductive reactance, which causes a severe dispersion and hence a signal distortion; see Telegrapher's equations. This has to be solved by iron tape or by load coils. It was not until the 20th century that message transmission speeds over transatlantic cables would reach even 120 words per minute. Despite this, London had become the world center in telecommunications. Eventually, no fewer than eleven cables radiated from Porthcurno Cable Station near Land's End and formed with their Commonwealth links a "live" girdle around the world.

Repeaters

The original cables were not fitted with repeaters, which would have amplified the signal along the way, because there was no practical way to power the relays. As technology advanced, intermediate relays became possible and the first transatlantic cable with repeaters was TAT-1 in 1956.

In fiction

Impact

A 2018 study in the American Economic Review found that the transatlantic telegraph substantially increased trade over the Atlantic and reduced prices.^[31] The study estimates that "the efficiency gains of the telegraph to be equivalent to 8 percent of export value".^[31]

See also

Notes

1. ^{{cite book |last= Moll|first=Marita |last2=Shade |first2= Leslie Regan |date= 2004 |title=Seeking Convergence in Policy and Practice: Communications in the Public Interest. Volume 2 |url=https://books.google.com/books?id=0f_LxEC2ZAQC&pg=PA27 |publisher=Canadian Center for Policy Alternatives |page= 27|isbn=0-88627-386-2}}
2. ^¹²{{Cite journal|last=Guarnieri|first=M.|year=2014|title=The Conquest of the Atlantic|journal=IEEE Industrial Electronics Magazine|volume=8|issue=1|pages=53–56/67|doi=10.1109/MIE.2014.2299492|ref=harv}}
3. ^{{cite web|last1=de Cogan|first1=D.|title=Dr E.O.W. Whitehouse and the 1858 trans-Atlantic Cable|url=http://atlantic-cable.com/Books/Whitehouse/DDC/index.htm|website=History of the Atlantic Cable & Undersea Communications|archiveurl=https://web.archive.org/web/20090906145610/http://atlantic-cable.com/Books/Whitehouse/DDC/index.htm|archivedate=2009-09-06}}
4. ^Wilson, Arthur (1994). The Living Rock: The Story of Metals Since Earliest Times and Their Impact on Civilization. p. 203. Woodhead Publishing. {{ISBN|978-1-85573-301-5}}.
5. ^{{cite book|author1=Henry Youle Hind|author2=Thomas C. Keefer|author3=John George Hodgins |author4= Charles Robb |title= Eighty Years' Progress of British North America: Showing the Wonderful Development of Its Natural Resources, Giving, in a Historical Form, the Vast Improvements Made in Agriculture, Commerce, and Trade, Modes of Travel and Transportation, Mining, and Educational Interests, Etc., Etc., with a Large Amount of Statistical Information, from the Best and Latest Authorities|url=https://books.google.com/books?id=dvodAAAAMAAJ&pg=PA759|year=1864|publisher=L. Stebbins|pages=759–}}
6. ^{{cite web |url=http://www.oxforddnb.com/index/101012824/ |title=Hearder, Jonathan Nash (1809–1876) |work=Oxford Dictionary of National Biography |last=Hearder |first=Ian G. |publisher=Oxford University Press |date=September 2004 |accessdate=7 April 2010}}
7. ^{{cite book |title=The Cable|first=Gillian|last=Cookson|page=73|year=2006|edition=2|publisher=Tempus Publishing|isbn=0-7524-3903-0}}
8. ^{{cite web|url = http://www.atlantic-cable.com/Books/1857Isaac/index.htm |title = History of the Atlantic Cable – Submarine Telegraphy – 1857 – Laying the Atlantic Telegraph From Ship To Shore |accessdate = August 5, 2008}}
9. ^{{cite web|url = http://www.atlantic-cable.com/Article/1858Leslies/index.htm |title = History of the Atlantic Cable – Submarine Telegraphy – Frank Leslie's Illustrated Newspaper 1858 Cable News |accessdate = August 5, 2008}}
10. ^{{cite web|url=http://thetelegraphfield.com|title=ザテレグラフフィールド|website=thetelegraphfield.com}}
11. ^{{cite web|url=http://www.alanryanhall.com|title=Alan Hall|publisher=}}
12. ^¹{{cite book|title=Report of the Joint Committee Appointed by the Lords of the Committee of Privy Council for Trade and the Atlantic Telegraph Company, to Inquire Into the Construction of Submarine Telegraph Cables: Together with the Minutes of Evidence and Appendix |chapterurl=https://books.google.com/books?id=76HmAAAAMAAJ&pg=PA232|accessdate=1 March 2018|year=1861|publisher=Eyre|location=Eyre and Spottiswoode|chapter=Manipulation of the Atlantic Telegraph Line. From August 10th to the 1st of September inclusive|pages=230–232}}
13. ^Harry Granick, Underneath New York, p. 115, Fordham University Press, 1991 {{ISBN|0823213129}}.
14. ^http://www.atlantic-cable.com/Article/Lanello/ The Curious Story of the Tiffany Cables. see 'The Great Transatlantic Cable – 1858." Bill Burns. Atlantic-cable.com. Retrieved June 12, 2014.
15. ^{{cite book |url=https://books.google.com/books?id=80sUAAAAYAAJ&pg=PA542&dq=%22The+Queen%27s+message+to+the+President+of+the+United+States,+Washington%22&hl=en&ei=UemZTpOHN8TrsgbunNydBA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CDIQ6AEwAQ#v=onepage&q&f=false |title='THE QUEEN'S MESSAGE' and 'THE PRESIDENT'S REPLY' (full wording) |work=History of the United States: from the earliest period to the administration of President Johnson |volume=Vol. 3 |author=Jesse Ames Spencer |publisher=Johnson, Fry |year=1866 |page=542, chap. 10 |format=Digitised eBook |accessdate=June 12, 2014}}
16. ^Jim Al-Khalili. "The Story of Electricity", Ep, 2 "The Age of Invention" October 13, 2011, BBC TV, Using Chief Engineer Bright's original notebook. Retrieved June 12, 2014
17. ^{{cite web |url=http://atlantic-cable.com/1858NY/index.htm |work=History of the Atlantic Cable & Undersea Communications |title=1858 NY Celebration |date=January 10, 2013 |publisher=Atlantic-cable.com |accessdate = August 5, 2008}}
18. ^{{cite book |last1=Mercer |first1=David |title=The Telephone: The Life Story of a Technology |date=2006 |publisher=Greenwood Publishing Group |isbn=9780313332074 |page=17 |url=https://books.google.cl/books?id=KUHgxpR5K4QC&pg=PA17 |accessdate=27 June 2018}}
19. ^Kieve, p. 109
20. ^Voice Across the Sea – Arthur C Clarke
21. ^Kieve, p. 109
22. ^{{cite web|url=https://books.google.com/books?id=UoxAwrVzc88C&pg=PA515&lpg=PA515&dq=transatlantic+cable+buoy&source=bl&ots=vM5O99xNRP&sig=3J7hvgSPZ2usC7pZwgw3-ZfJgo0&hl=en&ei=ugXKTcmWC-jkiAKmoaWjBQ&sa=X&oi=book_result&ct=result&resnum=5&ved=0CCwQ6AEwBDgK#v=onepage&q=transatlantic+cable+buoy&f=false|title=The Encyclopaedia Britannica: A Dictionary of Arts, Sciences, Literature and General Information|first=Hugh|last=Chisholm|date=March 18, 2018|publisher=Encyclopaedia britannica Company|via=Google Books}}
23. ^{{cite web|url=http://atlantic-cable.com/Article/1915CCC/index.htm|title=History of the Atlantic Cable & Submarine Telegraphy - 1915: How submarine cables are made and laid|website=atlantic-cable.com}}
24. ^{{cite web|url=http://www.atlantic-cable.com/Cableships/GreatEastern/|title=History of the Atlantic Cable & Submarine Telegraphy - Great Eastern|website=www.atlantic-cable.com}}
25. ^{{cite web|url = http://atlantic-cable.com/Article/Gooch/index.html |title = History of the Atlantic Cable – Submarine Telegraphy – Daniel Gooch |accessdate = August 5, 2008}}
26. ^http://atlantic-cable.com/CableCos/AngloAmerican/index.htm Anglo-American Telegraph Company
27. ^{{cite web|url = http://atlantic-cable.com/Article/1866Recovery/index.htm |title = History of the Atlantic Cable – Submarine Telegraphy-Recovery of the Lost Cable |accessdate = August 5, 2008}}
28. ^{{cite journal|title=Laying the French Atlantic Cable|journal=Nautical Magazine|year=1869|volume=38|url=https://books.google.com/books?id=7o4EAAAAQAAJ&pg=PA460|accessdate=September 10, 2011|page=460|publisher=Brown, Son and Ferguson|quote=To have navigated the ship in a fog so exactly to her proper position was certainly a most wonderful testimony to Captain Halpin's judgment and skill, ...}}
29. ^{{cite web |url=http://epe.lac-bac.gc.ca/100/205/301/ic/cdc/cable/fmessages.htm |work=Famous Messages Received and Sent |title=1858 August 16 Queen Victoria Sends Telegram to President Buchanan |publisher=Library and Archives Canada |accessdate = July 9, 2010}}
30. ^{{cite web |url=http://www.iscpc.org/information/Narative_Cable_History.htm |title=Narrative History Of Submarine Cables |publisher=International Cable Protection Committee |date= February 26, 2014 |accessdate=June 12, 2014}}
31. ^¹{{Cite journal|last=Steinwender|first=Claudia|date=2018|title=Real Effects of Information Frictions: When the States and the Kingdom Became United|url=https://www.aeaweb.org/articles?id=10.1257/aer.20150681|journal=American Economic Review|language=en|volume=108|issue=3|pages=657–696|doi=10.1257/aer.20150681|issn=0002-8282|via=}}