[One Sip of a Book] Understanding Units Reveals Science

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At the end of 2018, the International Bureau of Weights and Measures abolished the International Prototype of the Kilogram and completely revised the International System of Units based on seven defined constants existing in nature. This book provides a detailed explanation of the major units and constants based on the new International System of Units. It focuses on the seven defined constants that form the basis of the SI units, such as the speed of light, elementary charge, and Planck constant; the seven base units defined by these constants, including the second, meter, and kilogram; and 20 important derived units named after scientists, such as Newton (N), Joule (J), Watt (W), and Tesla (T). It also covers radiation-related units frequently encountered in the news, such as Becquerel (Bq), named after Henri Becquerel who first discovered radiation, and Sievert (Sv), named after Swedish physicist Rolf Maximilian Sievert who studied the harmful effects of radiation absorbed by the human body. The book explains not only the definitions and uses of units but also the concepts of physical quantities expressed by units, such as force, power, voltage, and magnetic field, along with the relevant scientific theories.

In addition to Tesla, the unit Gauss (G or Gs) is used to indicate the strength of a magnetic field. Gauss, named after Carl Friedrich Gauss, a German mathematician who discovered Gauss’s law related to electric fields and also studied the Earth's magnetism, is not included in the International System of Units but is more frequently used than Tesla in engineering fields. One Gauss equals 0.0001 Tesla. In other words, 10 Gauss equals 1 Tesla. (p. 202)

Since we constantly experience hot and cold in daily life, we understand well what it means to be hot or cold. However, scientifically explaining what causes the sensation of hotness or coldness is not so simple. Scientists debated for a long time over the nature of heat. It was only after the 19th century, when it became known that matter is composed of atoms and molecules, that the true nature of heat could be properly understood. (p. 229)

Feeling the need for accurate temperature measurement while conducting meteorological observations, he decided to create a thermometer based on natural phenomena. Celsius chose the boiling and freezing points of water as reference points. He made a mercury thermometer with the boiling point of water set at 0 degrees and the freezing point at 100 degrees. This happened in 1742, two years before he died of tuberculosis at the age of 42. The temperature scale Celsius created is called the Celsius scale because in China, Celsius was transcribed as Seobisa (攝爾思). (p. 247)

The dose equivalent, which considers the effects of radiation on the human body, is expressed using the unit Sievert (Sv). Sievert is named after Swedish physicist Rolf Maximilian Sievert, honoring his contributions to the study of the biological effects of ionizing radiation. The unit was named at the International Bureau of Weights and Measures conference held in 1979. Sievert (Sv) is calculated by multiplying the absorbed energy unit Gray (Gy) by a weighting factor determined according to the type of radiation. (p. 262)

Understanding Science Through Units | Written by Gwak Youngjik | Sero Books | 368 pages | 21,000 KRW