ρ | rho

The Greek letter ρ (rho) was derived from the ancient Phoenician letter 𐤓 (rēš), which is thought to have come from a pictogram of a head. As a numeral, ρ has a value of 100. Incidentally, this is our 100^th post.

The lower-case ρ (or its variant ϱ) has many uses in maths and sciences, such as:

• in electrical engineering and physics: resistivity (the reciprocal of conductivity σ)
• in electromagnetism: the volume charge density, ρ_q
• in molecular biology: the ρ factor, a protein involved in the termination of transcription
• in mathematics: the Dickman–de Bruijn function ρ(u)
• in mathematics: the plastic number, a constant with approximate value of 1.32472, the unique real solution of the cubic equation x³ = x + 1
• in mathematics: the prime constant, an irrational number with approximate value of 0.41468
• in mathematics: ρ(A), the spectral radius of a matrix A
• in organic chemistry: ρ-value (the reaction constant) of the Hammett equation
• in particle physics: rho mesons ρ⁺, ρ⁰ and ρ⁻
• in physics: mass density and air density
• in spectroscopy: rotational relaxation time
• in statistics: the Pearson correlation coefficient

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百 | bǎi

In Mandarin Chinese, 百 (Pinyin: bǎi 🔊; not to be confused with bā, 🔊) is a word for “hundred”. It is often used figuratively, in a sense “numerous”, “countless”, “every”, “all”, “entirely”, “at all”, etc. When they actually mean the number 100, “one hundred”, Chinese say exactly that: 一百 (yī bǎi 🔊).

There is a great variety of historical forms of this character. 百 is a phono-semantic compound where pronunciation of seemingly unrelated word 白 (bái) “white” is combined with meaning of 一. (Recall that, apart from “one”, 一 also means “each”, “every”, “whole” and so on). A good trick to remember this character is to write the number 100 with “boxy” zeros, then rotate it 90° clockwise.

Another hanzi with the same meaning, 佰, is mainly used for accounting purposes.

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