光钟为何可能取代铯钟成为秒的新基准？
Why might optical clocks replace cesium clocks as the new second?
光钟使用更高频率的光学跃迁，频率不确定度比铯钟低两个数量级以上，稳定性更好。国际计量界正推动以光频跃迁重新定义秒。
Optical clocks use higher-frequency optical transitions with uncertainties over a hundred times smaller than cesium, offering far better stability. The metrology community is moving to redefine the second by an optical transition.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#光钟 #原子钟 #秒重定义
量纲分析为什么能在不解方程的情况下猜出公式？
Why can dimensional analysis guess a formula without solving any equations?
物理规律必须在单位改变时保持形式，这个约束本身就大幅限制了变量的组合方式。例如只用绳长和重力加速度构造时间量纲，就唯一得到根号下长度除以重力，正是单摆周期的形式，只差一个无量纲系数。
A physical law must keep its form when units change, and that constraint alone sharply limits how variables can combine. From only length and gravity, the sole combination with the dimension of time is the square root of length over g — exactly the pendulum period up to a dimensionless factor.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#量纲分析 #标度 #估算
LIGO 如何探测到引力波？
How does LIGO detect gravitational waves?
LIGO 用公里级迈克尔逊激光干涉仪，引力波经过时使互相垂直的双臂长度产生极微小的相对变化（约质子直径的千分之一），通过激光相位差检测出来。2015年首次直接探测到黑洞并合产生的引力波。
LIGO uses kilometer-scale Michelson laser interferometers; a passing gravitational wave changes the relative arm lengths by tiny fractions — about a thousandth of a proton diameter — detected as laser phase shifts. In 2015 it first directly observed waves from a black-hole merger.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#引力波 #激光干涉 #LIGO
开尔文现在由玻尔兹曼常数定义，这跟“水的三相点”有什么不同？
The kelvin now rests on the Boltzmann constant — how does that differ from the triple point of water?
旧定义把水的三相点定为273.16 K，但真实水的同位素组成和杂质会影响这个点的复现精度。新定义直接固定玻尔兹曼常数，把温度化为能量单位的换算，任何能测量热运动能量的方法都可作为基准，摆脱了对特定物质的依赖。
The old definition assigned 273.16 K to water's triple point, yet isotopic composition and impurities limited how well that point could be reproduced. Fixing the Boltzmann constant instead makes temperature a conversion into energy units, so any measurement of thermal energy can realise the kelvin without relying on one substance.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 来源待核验 · Source pending review
来源 · Source
出处备注：著录含书名，但缺版次 / 出版社 / 年份中的一部分，需编辑补全后才能做独立比对。
BIPM第26届国际计量大会决议1（2018），《国际单位制手册》第9版附录2
BIPM, 26th CGPM Resolution 1 (2018), Annex 2 of the SI Brochure, 9th edition
#开尔文 #玻尔兹曼常数 #温标
一千克为什么不再是巴黎那块铂铱合金砝码了？
Why is the kilogram no longer a platinum-iridium cylinder in Paris?
实物基准会被污染、磨损，一百多年里与各国副本相差了几十微克，而且一旦损毁无法复原。2019年起，千克改由固定普朗克常数的数值来定义，通过基布尔天平把质量与电磁量和频率联系起来，任何实验室都能复现。
A physical artefact can be contaminated or worn, and it drifted tens of micrograms from its copies over a century, with no way to restore it if damaged. Since 2019 the kilogram is defined by fixing the Planck constant, linking mass to electrical quantities and frequency through a Kibble balance that any lab can reproduce.
难度 专业 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#千克 #普朗克常数 #基布尔天平
2019年千克为何改用普朗克常数来定义？
Why was the kilogram redefined via the Planck constant in 2019?
旧千克依赖巴黎的实物原器，会因污染损耗而漂移；2019年起千克通过固定普朗克常数并用基布尔秤把质量与频率和能量精确联系，使单位不再依赖实物。
The old kilogram depended on a physical artifact that drifted with contamination; since 2019 the kilogram fixes the Planck constant via a Kibble balance, linking mass to energy and frequency without a physical object.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#千克 #普朗克常数 #基布尔秤
精细结构常数 α≈1/137 为什么被视为物理学中深刻的基本常数？
Why is the fine-structure constant α≈1/137 regarded as a profound fundamental constant in physics?
精细结构常数 α≈1/137 是无量纲基本常数，决定电磁相互作用强度，出现在原子能级精细结构中。其数值无法由理论推导，只能由实验精密测量。
The fine-structure constant α≈1/137 is a dimensionless fundamental constant setting the strength of electromagnetism and appearing in atomic fine structure. Its value cannot be derived but only measured precisely.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 来源待核验 · Source pending review
来源 · Source
出处备注：著录仅给书名，无版次与年份；同一书名常对应多个版次，不臆测具体版本。
《现代物理学评论》— 基本常数测定
Reviews of Modern Physics — determination of fundamental constants
#精细结构常数 #基本常数 #无量纲
千克改为依据普朗克常数定义之后，基布尔天平通过怎样的两阶段操作把一个实物砝码的质量与普朗克常数联系起来？
After the kilogram became tied to the Planck constant, how does the two-phase operation of a Kibble balance connect the mass of a physical test weight to that constant?
第一阶段为称量模式：线圈载流后在磁场中受到的洛伦兹力恰好抵消砝码重力。第二阶段为速度模式：让线圈以恒定速度在同一磁场中运动并记录感应电动势。两式相乘消去线圈与磁场的几何因子，机械功率便与电压、电阻挂钩，而这些电学量经由约瑟夫森效应和量子化霍尔效应溯源至普朗克常数与基本电荷，最终质量的不确定度可达10^-8量级。
In the weighing phase, the Lorentz force on a current-carrying coil suspended in a magnetic field exactly balances the weight of the test mass. In the velocity phase, the coil is driven at constant speed through the same field while the induced voltage is recorded. Multiplying the two measurements cancels the coil-and-field geometry factor, linking mechanical power to voltage and resistance, which are themselves realized through the Josephson effect and the quantum Hall effect against the Planck constant and the elementary charge; relative uncertainties reach the 10^-8 level.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 来源待核验 · Source pending review
来源 · Source
出处备注：著录含书名，但缺版次 / 出版社 / 年份中的一部分，需编辑补全后才能做独立比对。
国际计量局（BIPM）《国际单位制手册》第9版（2019）附录2中关于千克实用计量的实现文件
BIPM, The International System of Units (SI Brochure), 9th ed. (2019), Appendix 2 on the mise en pratique of the kilogram
#基布尔天平 #量子计量
如何用完美硅球测量阿伏伽德罗常数？
How can a perfect silicon sphere be used to measure Avogadro's number?
通过测量单晶硅球的直径、质量和晶格常数，算出单位体积原子数，从而得到每摩尔的原子数（阿伏伽德罗常数），为物质的量单位提供独立测定路径。
By measuring a single-crystal silicon sphere's diameter, mass, and lattice spacing, one counts atoms per volume and hence atoms per mole — Avogadro's number — giving an independent route to the mole.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#阿伏伽德罗常数 #硅球 #物质的量
摄氏度和开尔文的“一度”为什么正好一样大？
Why is one degree Celsius exactly the same size as one kelvin?
摄氏度和开尔文的单位大小完全相同，因为开尔文的定义就是“1 开尔文 = 1 摄氏度”，两者只差一个固定偏移（0 °C = 273.15 K）。因此温度变化 1 °C 恰好等于变化 1 K，二者度量的是同一段温度间隔。只有零点位置不同，度的“大小”并没有变。
The Celsius and kelvin scales use exactly the same unit size because the kelvin was defined so that one kelvin equals one degree Celsius; they differ only by an offset (0 °C = 273.15 K). A change of 1 °C is therefore exactly a change of 1 K, since both measure the same temperature interval. Only the zero point is shifted, not the size of a degree.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#摄氏度 #开尔文 #温标
卡文迪许实验如何首次「称量」地球？
How did the Cavendish experiment first 'weigh' the Earth?
卡文迪许用扭秤测量两个小球间的微弱引力，从而确定万有引力常数G，再结合地表重力与地球半径即可算出地球质量，故被称为称量地球。
Cavendish used a torsion balance to measure the tiny force between lead balls, determining G; with surface gravity and Earth's radius he computed Earth's mass, hence 'weighing the Earth'.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-10-03 · 2 条独立证据
来源 · Source
《费曼物理学讲义》第一卷
The Feynman Lectures on Physics, Volume I
独立核验依据 · Independent verification: 核验依据 2 ISBN 978-0-7382-0298-3 (2006 Pearson 版，原始版为1963-65 Addis
#卡文迪许实验 #万有引力常数 #地球质量
一米现在是怎么定义的？
How is the metre defined today?
先固定真空中光速为299792458米每秒，再规定一米就是光在1/299792458秒内走过的距离。因为秒可以用原子钟极精确地实现，长度基准也就跟着变得极其稳定。
The speed of light in vacuum is fixed at 299792458 m/s, and the metre is then the distance light travels in 1/299792458 of a second. Since the second is realised superbly well by atomic clocks, the length standard inherits that stability.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#米 #光速 #单位定义
国际单位制在2019年把“千克”改由普朗克常数定义，为什么不再用巴黎那块铂铱合金砝码？
In 2019 the SI redefined the kilogram via the Planck constant—why abandon the platinum–iridium cylinder in Paris?
那块国际千克原器会随时间微量失重或沾污，且全世界都要依赖它，风险集中。新定义用基布尔秤把质量与普朗克常数 h 及精确固定的电学单位挂钩，使千克可由自然常数复现，任何实验室都能自洽地得到，不再依赖单一实物。
The international prototype could lose or gain mass subtly over time and put all the world's mass standard in one object. The new definition links mass to the Planck constant h via a Kibble balance and fixed electrical units, so the kilogram is realized from natural constants and can be reproduced in any lab without a single artifact.
难度 进阶 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-11-05 · 3 条独立证据
来源 · Source
BIPM,《国际单位制(SI)》(第9版,2019),附录(千克新定义)
BIPM, 'The International System of Units (SI)' (9th ed., 2019), Appendix (redefinition of the kilogram)
独立核验依据 · Independent verification: 核验依据 1 ISBN 9787502650162（豆瓣书目条目: 新国际单位制） · 核验依据 3 Crossref DOI 10.59161/auez1291
#国际单位制 #普朗克常数 #千克定义
现在「一秒」是如何定义的？
How is one second currently defined?
国际单位制中，秒定义为铯-133原子基态超精细能级跃迁频率的9192631770个周期所持续的时间，由铯原子钟实现。
In SI, the second is defined as the duration of 9,192,631,770 periods of the radiation from the hyperfine transition of cesium-133, realized by cesium atomic clocks.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#秒 #铯原子钟 #SI定义
现在「米」是如何定义的？
How is the meter currently defined?
米定义为光在真空中于 1/299792458 秒内传播的距离，把真空光速c固定为精确值，长度单位由时间和光速导出。
The meter is the distance light travels in vacuum in 1/299,792,458 of a second, fixing the speed of light c as exact; length derives from time and c.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#米 #光速 #SI定义
“秒”现在是用什么物理过程来定义的，为什么偏偏选铯-133原子的某种跳动？
What physical process now defines the 'second,' and why specifically a certain transition of cesium-133?
自 1967 年起，1 秒被定义为铯-133原子基态超精细能级间跃迁辐射的 9,192,631,770 个周期。这一微波频率极其稳定且可复现，不受地点、环境实物影响，因此成为最精确的时间基准。现代原子钟就是数这个跳动来计时的。
Since 1967 the second is defined as 9,192,631,770 periods of the radiation from the hyperfine transition of cesium-133. This microwave frequency is extremely stable and reproducible, independent of place or artifacts, making it the most precise time base. Modern atomic clocks literally count this ticking.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-11-05 · 3 条独立证据
来源 · Source
BIPM,《国际单位制(SI)》(第9版,2019),时间单位(秒)
BIPM, 'The International System of Units (SI)' (9th ed., 2019), Unit of time (second)
独立核验依据 · Independent verification: 核验依据 1 OPENLIBRARY 标识 /works/OL13574224W：The international syst · 核验依据 3 Crossref DOI 10.59161/auez1291
#秒定义 #铯-133 #超精细结构
一秒为什么要用铯原子来定义，而不是地球自转？
Why define the second with a caesium atom rather than Earth's rotation?
地球自转会因潮汐摩擦、地核运动和大气变化而不均匀，一天的长度每年都在微小地改变。铯-133基态两个超精细能级之间的跃迁频率恒定且可复现，规定其为9192631770周即为一秒，稳定性高出许多个数量级。
Earth's spin is uneven because of tidal friction, core motion and atmospheric shifts, so the length of a day wobbles year to year. The hyperfine transition of caesium-133 is constant and reproducible, so the second is defined as 9192631770 of its cycles — many orders of magnitude steadier.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#秒 #铯原子 #超精细能级
基布尔天平凭什么能把千克的质量基准挂到普朗克常数这样一个量子量上？
By what reasoning does the Kibble balance tie the kilogram, a macroscopic mass artifact, to the quantum quantity Planck's constant?
仪器分两步运行：先用通电线圈所受洛伦兹力抵消砝码重力，再让线圈匀速运动测量感应电动势。借助约瑟夫森电压标准和量子化霍尔电阻标准，两步里的电学量都可写成频率乘以普朗克常数的形式，速度项消去重力与几何因子后质量便唯一地由h确定。2019年修订的国际单位制据此按固定值h = 6.62607015×10⁻³⁴ J·s复现千克。
The instrument runs in two phases: it first cancels a test mass's weight with the Lorentz force on a current-carrying coil, then measures the voltage induced when the coil moves at constant velocity. Using the Josephson voltage standard and the quantum Hall resistance standard, both electrical quantities reduce to frequency multiplied by Planck's constant, so after velocity terms cancel gravity and geometric factors the mass fixes itself uniquely to h. The revised SI of 2019 realizes the kilogram from the fixed value h = 6.62607015×10⁻³⁴ J·s.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 来源待核验 · Source pending review
来源 · Source
出处备注：著录含书名，但缺版次 / 出版社 / 年份中的一部分，需编辑补全后才能做独立比对。
BIPM第26届国际计量大会决议1（2018），《国际单位制手册》第9版附录2
BIPM, 26th CGPM Resolution 1 (2018), Annex 2 of the SI Brochure, 9th edition
#千克重新定义 #量子计量
如果工程师忘记修正相对论,GPS 每天会错出多远?
If engineers forgot relativistic corrections, how far would GPS drift per day?
两个效应方向相反都要算。狭义相对论:卫星时速近1.4万公里,星载钟比地面慢约每天7微秒;广义相对论:轨道高度引力较弱,星载钟快约每天45微秒;净效果是快38微秒。导航定位依赖纳秒级时间测量——38微秒对应约11公里的测距误差,而且逐日累积,不出几分钟定位就不可信。因此卫星出厂前把时钟频率预先调低,入轨后正好走准;接收机还要实时扣除信号穿过电离层与大气折射的延迟。GPS 是普通人每天都在用的相对论实验。
Two opposite effects must both be booked. Special relativity slows satellite clocks about seven microseconds daily at fourteen thousand km/h; general relativity speeds them up about forty-five microseconds in weaker gravity; net gain thirty-eight microseconds. Ranging needs nanosecond timing, so thirty-eight microseconds maps to roughly eleven kilometers of error per day — useless within minutes without correction. Satellites therefore fly with factory-prelowered clock frequencies that land exactly right on orbit, and receivers still strip out ionospheric and tropospheric delays. GPS is a relativistic experiment you use daily.
难度 进阶 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-11-02
来源 · Source
Ashby,《Relativity in the Global Positioning System》,Living Reviews in Relativity 第6卷,2003
N. Ashby, Relativity in the Global Positioning System, Living Reviews in Relativity 6, 2003
独立核验依据 · Independent verification: 核验依据 1 CROSSREF 标识 10.12942/lrr-2003-1：Relativity in the Global
#GPS #时间膨胀 #相对论修正
计算结果为什么不能想写多少位小数就写多少位？
Why can't you keep as many decimal places as your calculator shows?
计算结果不可能比输入数据中最不精确的那一项更准确，多写小数位只是凭空制造了数据并不支持的“精确感”。有效数字规则让答案如实反映测量不确定度，而不是把计算器产生的多余数字当成真实精度。用两位小数测量得到的量，却报出五位小数，是误导人的。
A result can only be as precise as the least-precise measurement that went into it; extra decimal places imply a certainty the data don't support. Significant-figure rules keep the answer honest about uncertainty instead of letting calculator noise masquerade as real precision. Reporting five decimal places when your input was measured to two digits is misleading.
难度 入门 · 适合 小学高段(9-12)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#有效数字 #精度 #计算
科学家为什么爱写 3.0×10⁸ 而不是 300000000？
Why do scientists write 3.0×10⁸ instead of 300000000?
科学记数法把数量级和有效数字分开，既不容易数错零，也一眼看出精度。3.0×10⁸ 表示只有两位有效数字可信，而写一长串零反而暗示后面每一位都精确。
Scientific notation separates the order of magnitude from the significant figures, so you neither miscount zeros nor lose sight of precision. Writing 3.0×10⁸ says only two digits are trustworthy, while a long string of zeros wrongly implies every digit is exact.
难度 入门 · 适合 小学高段(9-12)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#科学记数法 #有效数字 #数量级
物理实验为什么每个测量结果都要写上不确定度？
Why must every measured result carry an uncertainty?
任何测量都有仪器分辨率、环境波动和读数偏差，单个数字无法说明它可信到什么程度。写成“1.24±0.03秒”才能让别人判断两组结果是否真的矛盾，也是重复实验和理论比较的前提。
Every measurement is limited by instrument resolution, environmental drift and reading bias, so a bare number says nothing about how far to trust it. Reporting 1.24 ± 0.03 s lets others judge whether two results genuinely disagree, which is essential for replication and comparison with theory.
难度 入门 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-08-23 · 2 条独立证据
来源 · Source
程守洙 江之永,《普通物理学》第七版,高等教育出版社,2016
Cheng Shouzhu and Jiang Zhiyong, General Physics, 7th ed., Higher Education Press, 2016
独立核验依据 · Independent verification: 核验依据 2 ISBN 9787040429190（豆瓣书目条目: 普通物理学．上册）
#不确定度 #误差 #测量