Thermometer with Fahrenheit and Celsius units

Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit (1686–1736).^[1] Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees. The Fahrenheit scale was replaced by the Celsius scale in most countries during the mid to late 20th century,^[2] though Canada retains it as a supplementary scale that can be used alongside Celsius.^[3][4][5][6] Fahrenheit remains the official scale of the United States, Cayman Islands and Belize.^[7] The Rankine temperature scale was based upon the Fahrenheit temperature scale, with its zero representing absolute zero instead.

Definition and conversions[link]

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F (at standard atmospheric pressure). This puts the boiling and freezing points of water exactly 180 degrees apart.^[8] Therefore, a degree on the Fahrenheit scale is ¹⁄₁₈₀ of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart. A temperature interval of 1 °F is equal to an interval of ⁵⁄₉ degrees Celsius. The Fahrenheit and Celsius scales intersect at −40° (−40 °F and −40 °C represent the same temperature).

Absolute zero is defined as −273.15 °C or −459.67 °F. The Rankine temperature scale was created to use degree intervals the same size as those of the Fahrenheit scale, such that a temperature difference of one degree Rankine (1 °R) is equal to a difference of 1 °F, except that absolute zero is 0 °R – the same way that the Kelvin temperature scale matches the Celsius scale, except that absolute zero is 0 K.^[8] The Fahrenheit scale uses (in the same manner as the later Celsius scale) the symbol ° to denote a point on the temperature scale and the letter F to indicate the use of the Fahrenheit scale (e.g. "Gallium melts at 85.5763 °F"),^[9] as well as to denote a difference between temperatures or an uncertainty in temperature (e.g. "The output of the heat exchanger experiences an increase of 72 °F" and "Our standard uncertainty is ±5 °F").

History[link]

Daniel Gabriel Fahrenheit

According to an article Fahrenheit wrote in 1724, he based his scale on three reference points of temperature.^[8] In his initial scale (which is not the final Fahrenheit scale), the zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt, at a 1:1:1 ratio. This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point, at 32 degrees, was a mixture of ice and water without the ammonium chloride at a 1:1 ratio. The third point, 96 degrees, was approximately the human body temperature, then called "blood-heat".^[10]

According to a letter Fahrenheit wrote to his friend Herman Boerhaave,^[11] his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer's scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale. He then re-calibrated his scale using the melting point of ice and normal human body temperature (which were at 30 and 90 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees and body temperature 96 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval six times (since 64 is 2 to the sixth power).^[10][12]

Fahrenheit observed that water boils at about 212 degrees using this scale. Later, other scientists decided to redefine the degree slightly to make the freezing point exactly 32 °F, and the boiling point exactly 212 °F or 180 degrees higher.^{[citation needed]} It is for this reason that normal human body temperature is approximately 98° (oral temperature) on the revised scale (whereas it was 90° on Fahrenheit's multiplication of Rømer, and 96° on his original scale).^[13]

Usage[link]

The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in English-speaking countries until the 1960s. In the late 1960s and 1970s, the Celsius scale (known until 1948 as centigrade) replaced Fahrenheit in many countries, typically during their metrication process.^[14]

Fahrenheit is used in the United States, Belize, and the United States territories of Puerto Rico, Guam and the U.S. Virgin Islands^[7] for everyday applications. For example, U.S. weather forecasts, food cooking, and freezing temperatures are typically given in degrees Fahrenheit. Scientists, such as meteorologists, use Celsius or Kelvin in all countries.^[15] In some nations, both measures are quoted.^[16]

Various reasons are given^[who?] for the resistance to the Celsius system in the U.S., including the larger size of each degree Celsius (resulting in the need for decimals where integer Fahrenheit degrees were adequate for much non-technical work).^{[citation needed]} Another reason is the lower zero point in the Fahrenheit system which reduces the number of negative signs when measurements such as weather data were averaged.^[17]

In some countries, Fahrenheit may be used in daily life alongside Celsius. For example, Canada has passed legislation favouring the International System of Units, while also maintaining legal definitions for traditional Canadian imperial units.^[18] Canadian weather reports are conveyed using degrees Celsius with occasional reference to Fahrenheit especially for trans border broadcasts. Virtually all Canadian ovens make legal use of the Fahrenheit scale.^[19] Thermometers, both digital and analogue, sold in Canada usually employ both the Celsius and Fahrenheit scales.^[20][21][22]

Fahrenheit is also utilized in the United Kingdom alongside Celsius. Weather reports displayed in newspapers and online quote temperatures in both Celsius and Fahrenheit (or have an option to enable this), especially during record-breaking weather.^[23] However, TV weather reports are primarily conveyed using degrees Celsius. Cooking instructions on most packaged food also list dual temperature scales along with digital/analogue thermometers.

Unicode representation of symbol[link]

The Fahrenheit symbol has its own Unicode character: "℉"(U+2109). This is a compatibility character encoded for roundtrip compatibility with legacy CJK encodings (which included it to conform to layout in square ideographic character cells) and vertical layout. Use of compatibility characters is discouraged by the Unicode Consortium. The ordinary degree sign (U+00B0) followed by the Latin letter F ("°F") is thus the preferred way of recording the symbol for degree Fahrenheit.

See also[link]

• Comparison of temperature scales

References[link]

External links[link]

Look up fahrenheit in Wiktionary, the free dictionary.

• NOAA web site: Fahrenheit to Celsius converter

Li You (李祐) (died June 22, 829^[1][2]), courtesy name Qingzhi (慶之), was a general of the Chinese dynasty Tang Dynasty. He started his career serving under the warlord Wu Yuanji but, after his capture by the imperial general Li Su in Li Su's campaign against Wu, became instrumental in Li Su's victory over Wu. He subsequently served the imperial government until his death immediately following his campaign against the warlord Li Tongjie.

Background and service under Wu Yuanji[link]

Little is known about Li You's background, including when he was born or where his family was from. It is known that he came to serve as an officer under Wu Yuanji, who controlled Zhangyi Circuit (彰義, headquartered in modern Zhumadian, Henan), and fought in Wu's campaign of resistance against the imperial government after the imperial government refused to sanction Wu Yuanji's takeover of the circuit after the death of his father Wu Shaoyang in 814. It was said that Li You was fierce in battle, and that the imperial forces feared him. He was also said to have inflicted many casualties on the imperial forces during the campaign.^[3][4]

Participation in Wu Yuanji's defeat[link]

By 817, one of the imperially-controlled circuits that was near Zhangyi and which thus continuously battled Zhangyi forces in the campaign, Tangsuideng Circuit (唐隨鄧, headquartered in modern Zhumadian as well), had seen two military governors (Jiedushi) — Gao Xiayu (高霞寓) and Yuan Zi — defeated by Zhangyi forces in rapid succession; in response, then-reigning Emperor Xianzong commissioned the less-known general Li Su as Tangsuideng's military governor.^[4][5] One of Li Su's tactics, while battling Zhangyi forces, was to capture Zhangyi officers that he considered capable, endearing them, and retaining them on his staff so that he could learn intelligence about Zhangyi. After he first captured Ding Shiliang (丁士良), Ding suggested that if he targeted and captured Chen Guangqia (陳光洽), the strategist for the key Zhangyi officer Wu Xiulin (吳秀琳), Wu would surrender — and after Ding captured Chen, Wu indeed surrendered. Li Su endeared himself to Wu and asked for Wu's input on how to capture Zhangyi's capital Cai Prefecture (蔡州). Wu responded, "If you, Lord, want to capture Cai Prefecture, you need Li You. I, Wu Xiulin, am not sufficiently capable."^[4]

Li Su thereafter targeted Li You and, after setting a trap for Li You with 300 cavalry soldiers while engaging him, Li Su's officer Shi Yongcheng (史用誠) was in fact able to capture Li You. Li Su's soldiers, resentful at how many casualties Li You had inflicted on them, demanded that Li You be put to death, but Li Su resisted the call and treated Li You with respect, causing Li You to be touched. It was said that when discussing tactics about how to capture Cai Prefecture, Li Su would discuss only with Li You and a fellow Zhangyi captive officer, Li Zhongyi (李忠義). The other officers feared that Li You would, at least meetings turn against Li Su and assassinate him, but Li Su did not heed their warnings. Thereafter, there were reports to Emperor Xianzong by those jealous of Li You that Li You must have been plotting still with the Zhangyi rebels, and there were renewed calls for Li You's execution. Li Su, after holding Li You and weeping, publicly declared that he would send Li You to Emperor Xianzong for Emperor Xianzong to execute him — but sent a secret petition to Emperor Xianzong, stating that if Li You were executed, the campaign would fail. Emperor Xianzong, after reviewing the petition, released Li You and returned him to Li Su. Li Su subsequently made Li You his guard commander^[4]

Li You thereafter informed Li Su that Zhangyi's elite forces were all stationed at Huiqu (洄曲, in modern Luohe, Henan), under the command of the officer Dong Chongzhi (董重質), and that Cai Prefecture itself was defended by old or weakened soldiers. He suggested that a surprise attack be made on Cai Prefecture itself to capture it, before the Huiqu forces would have any chance to react. After receiving approval from the chancellor Pei Du, whom Emperor Xianzong had put in charge overseeing the operations against Zhangyi, Li Su launched his surprise attack in winter 817. With Li You and Li Zhongyi serving as his forward commanders, he quickly reached Cai Prefecture during a winter storm, and with little resistance captured the city, seizing Wu Yuanji. The remaining Zhangyi forces under Dong surrendered, and the campaign was over. For his contributions during the campaign, Li You was made a general of the imperial guards.^[4]

Between the campaigns against Wu Yuanji and against Li Tongjie[link]

Li You was subsequently made the military governor of Xiasui Circuit (夏綏, headquartered in modern Yulin, Shaanxi) as well as the prefect of its capital Xia Prefecture (夏州).^[3] In 824, by which time Emperor Xianzong's grandson Emperor Jingzong was emperor, Li You was recalled to the capital Chang'an to serve as a general of the imperial guards. When he got to Chang'an, he made a tribute of 150 horses to Emperor Jingzong. The imperial censor Wen Zao (溫造) submitted an accusation that the tribute was improper and requested that Li You be arrested. Emperor Jingzong ordered that no actions be taken against Li You, but Li You still commented to his friends, "My heart was not at all shaken when I entered Cai Prefecture at midnight to capture Wu Yuanji, but now my gall falls before Censor Wen."^[6] Thereafter, because there was a Tufan incursion, Li You was sent to the front to serve as the military governor of Jingyuan Circuit (涇原, headquartered in modern Pingliang, Gansu).^[3]

Campaign against Li Tongjie[link]

As of 828, by which time Emperor Jingzong's younger brother Emperor Wenzong was emperor, the imperial forces were locked into a campaign against Li Tongjie, who had seized control of Henghai Circuit (橫海, headquartered in modern Cangzhou, Hebei) after the death of his father Li Quanlüe (李全略), the previous military governor. During the campaign, the imperial government had named a number of generals as military governors for Henghai, but each of whom either died during the campaign or was removed for other reasons. In winter 828, after the last of those, Fu Liangbi (傅良弼), died on the way to the Henghai front, Li You was named the military governor of Henghai and sent to the front.^[6]

In spring 829, when 3,000 Yicheng Circuit (義成, headquartered in modern Anyang, Henan) soldiers, who were fighting Henghai forces, suddenly mutinied and deserted, Li You intercepted them and slaughtered them. He subsequently defeated Li Tongjie in a battle and then attacked one of Henghai's prefectures, De Prefecture (德州, in modern Dezhou, Shandong). He quickly captured De Prefecture, and with Henghai's capital Cang Prefecture (滄州) under attack by a general under imperial orders, Li Zaiyi the military governor of Lulong Circuit (盧龍, headquartered in modern Beijing), the De Prefecture soldiers fled to Zhen Prefecture (鎮州, in modern Shijiazhuang, Hebei), the headquarters of Li Tongjie's ally Wang Tingcou the military governor of Chengde Circuit (成德).^[7]

Li Tongjie, knowing that his defeat was at hand, sent messengers to Li You, offering to surrender. Li You agreed and submitted Li Tongjie's offer to surrender to the imperial government, while sending his officer Wan Hong (萬洪) to Cang Prefecture to take control of the city from Li Tongjie. The imperial officer Bo Qi (柏耆) was sent to Cang Prefecture to comfort the region, but Bo, fearing that Li Tongjie was trying to trick imperial forces, led several hundred cavalry soldiers himself into Cang Prefecture. He found an excuse to kill Wan, and then had Li Tongjie and his families arrested and bound, to be delivered to Chang'an. On the way of delivering Li Tongjie to Chang'an, he heard rumors that Wang was intending to try to rescue Li Tongjie, so he beheaded Li Tongjie and delivered just the head to Chang'an.^[7]

By this point, Li You was ill, and when he heard the news of Wan's death, he was so shocked that his conditions fell worse. Meanwhile, various generals also submitted accusations against Bo, for having killed Wan for no reason and for appearing to want to take the credits of defeating Li Tongjie for himself. Emperor Wenzong commented, "If Li You dies, it is Bo Qi who kills him!" When Li You did die on June 22,^[1] Emperor Wenzong ordered Bo to commit suicide.^[7]

Notes and references[link]

1. ^ ^{a b} Academia Sinica Chinese-Western Calendar Converter.
2. ^ Book of Tang, vol. 17, part 1.
3. ^ ^{a b c} Book of Tang, vol. 161.
4. ^ ^{a b c d e} Zizhi Tongjian, vol. 240.
5. ^ Zizhi Tongjian, vol. 239.
6. ^ ^{a b} Zizhi Tongjian, vol. 243.
7. ^ ^{a b c} Zizhi Tongjian, vol. 244.

• Book of Tang, vol. 161.
• New Book of Tang, vol. 214.
• Zizhi Tongjian, vols. 240, 243, 244.