Editing Resistor (section)

===Preferred values===
{{See also|E-series of preferred numbers}}
Early resistors were made in more or less arbitrary round numbers; a series might have 100, 125, 150, 200, 300, etc.<ref>{{cite web |title=1940 Catalog – page 60 – Resistors |url=http://www.radioshackcatalogs.com/html/1940/hr060.html |website=[[RadioShack]] |access-date=11 July 2017 |archive-url=https://web.archive.org/web/20170711160604/http://www.radioshackcatalogs.com/html/1940/hr060.html |archive-date=11 July 2017}}</ref> Early power wirewound resistors, such as brown vitreous-enameled types, were made with a system of preferred values like some of those mentioned here. Resistors as manufactured are subject to a certain percentage [[Engineering tolerance|tolerance]], and it makes sense to manufacture values that correlate with the tolerance, so that the actual value of a resistor overlaps slightly with its neighbors. Wider spacing leaves gaps; narrower spacing increases manufacturing and inventory costs to provide resistors that are more or less interchangeable.

A logical scheme is to produce resistors in a range of values which increase in a [[geometric progression]], so that each value is greater than its predecessor by a fixed multiplier or percentage, chosen to match the tolerance of the range. For example, for a tolerance of ±20% it makes sense to have each resistor about 1.5 times its predecessor, covering a decade in 6 values. More precisely, the factor used is 1.4678 ≈ <math>10^{1/6}</math>, giving values of 1.47, 2.15, 3.16, 4.64, 6.81, 10 for the 1–10-decade (a decade is a range increasing by a factor of 10; 0.1–1 and 10–100 are other examples); these are rounded in practice to 1.5, 2.2, 3.3, 4.7, 6.8, 10; followed by 15, 22, 33, ... and preceded by ... 0.47, 0.68, 1. This scheme has been adopted as the [[E6 (number series)|E6 series]] of the [[International Electrotechnical Commission|IEC]] 60063 [[preferred number]] values. There are also '''E12''', '''E24''', '''E48''', '''E96''' and '''E192''' series for components of progressively finer resolution, with 12, 24, 48, 96, and 192 different values within each decade. The actual values used are in the [[International Electrotechnical Commission|IEC]] 60063 lists of preferred numbers.

A resistor of 100 ohms ±20% would be expected to have a value between 80 and 120 ohms; its E6 neighbors are 68 (54–82) and 150 (120–180) ohms.  A sensible spacing, E6 is used for ±20% components; E12 for ±10%; E24 for ±5%; E48 for ±2%, E96 for ±1%; E192 for ±0.5% or better. Resistors are manufactured in values from a few milliohms to about a gigaohm in IEC60063 ranges appropriate for their tolerance. Manufacturers may sort resistors into tolerance-classes based on measurement.  Accordingly, a selection of 100 ohms resistors with a tolerance of ±10%, might not lie just around 100 ohm (but no more than 10% off) as one would expect (a bell-curve), but rather be in two groups – either between 5 and 10% too high or 5 to 10% too low (but not closer to 100 ohm than that) because any resistors the factory had measured as being less than 5% off would have been marked and sold as resistors with only ±5% tolerance or better.  When designing a circuit, this may become a consideration.  This process of sorting parts based on post-production measurement is known as "binning", and can be applied to other components than resistors (such as speed grades for CPUs).