为什么电阻阻值不是整数？
如果电阻阻值像人民币一样，有1Ω、2Ω、5Ω、10Ω、20Ω、50Ω、100Ω、200Ω、500Ω……那该多好，因为这样我们就可以配出任意阻值来用了呀。
但看看上表实际中的电阻值，杂七杂八的，什么25.5kΩ、1.3kΩ，这些奇怪的数值用来干嘛？
师傅说过：“你不懂，这跟电阻制作工艺有关，电阻厂商是想做50Ω的，但做出来测量一下，发现是51Ω，所以就有了这个阻值，每批物料的阻值都是不一样的。”
这听起来有道理，可是1.82kΩ又是怎么回事？25.5kΩ呢？每批物料的阻值真的都是不一样的吗？电阻制作工艺又是怎么回事？
师傅有着如同诗人般的直觉，任何问题他都可以直接作答，你永远不会知道这个答案从何而来；他有着艺术家般的想象力，任何矛盾与不明确都不会吸引他的注意，他的答案经常会发散到无法验证的境界；更厉害的是，多年的工程经验以及职业的训练，让他善于把这些浪漫的故事加上工程的语言，把猜想伪装成结论。他是很有本事的，但这并不足以解决我的疑惑。
下面转载一篇文章，又来解释这个问题，可以看出真正的工程师与诗人艺术家的答案完全不同。
Standard EIA Decade Resistor Values Table
The Electronic Industries Association (EIA), and other authorities, specify standard values for resistors, sometimes referred to as the “preferred value” system. The preferred value system has its origins in the early years of the last century at a time when most resistors were carbon-graphite with relatively poor manufacturing tolerances. The rationale is simple – select values for components based on the tolerances with which they are able to be manufactured. Using 10% tolerance devices as an example, suppose that the first preferred value is 100 ohms. It makes little sense to produce a 105 ohm resistor since 105 ohms falls within the 10% tolerance range of the 100 ohm resistor. The next reasonable value is 120 ohms because the 100 ohm resistor with a 10% tolerance is expected to have a value somewhere between 90 and 110 ohms. The 120 ohm resistor has a value ranging between 110 and 130 ohms. Following this logic, the preferred values for 10% tolerance resistors between 100 and 1,000 ohms would be 100, 120, 150, 180, 220, 270, 330 and so on (rounded appropriately); this is the E12 series shown in the table below.
The EIA “E” series specify the preferred values for various tolerances. The number following the “E” specifies the number of logarithmic steps per decade. The table below is normalized for the decade between 100 and 1,000. The values in any decade can be derived by merely dividing or multiplying the table entries by powers of 10. The series are as follows:
E3 50% tolerance (no longer used)
E6 20% tolerance (now seldom used)
E12 10% tolerance
E24 5% tolerance
E48 2% tolerance
E96 1% tolerance
E192 0.5, 0.25, 0.1% and higher tolerances
While the “E” preferred value lists are the best way to insure one is stocking the optimum number of values for a given tolerance, a word of caution is in order with respect to what is actually available in the marketplace and certain real world practices. For instance, the E48 list is often used as a stock list for 1% resistors for inventory control (48 values per decade rather than 96), but this practice leaves “holes” or gaps in one’s stock not covered by tolerance overlap, an undesirable practice in a prototype lab (less of an issue to the digital designer than to an analog circuit designer). The use of the E48 list for inventory control of 1% resistors works out well because every value on the E48 list just happens to also appear on the E96 list; the holes are thus symmetrical and easily filled by acquisition of one of the other 48 values per decade being omitted from stock. However, this is not always the case as can be seen by comparing the E24 and E96 lists. Nevertheless, many manufacturers make every single value on the E24 list in 1% tolerance even though the practice makes little mathematical sense (think about the obvious tolerance overlap between the 120 and 121 values for instance). Stocking only the E24 series in 1% will result in less symmetrical holes in stock than the practice of stocking only the E48 series. In any event, one should be aware of these practices to avoid confusion.
原文来自：LOGWELL