Historical question û origin of EIA resistor (mistaken?) values

[jims99]

We all know the standard (5%, 10%, or 20%) EIA resistor values. For example, the E12 series (10%) are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82.

These values were supposed to have been derived from the mathematical series of equally spacing values logarithmically for each decade(*). When calculating to arrive to values, we get following (rounded off to 2 significant digits: 10, 12, 15, 18, 22, 26, 32, 38, 47, 56, 68 and 83. Note the discrepancy for 5 out of 12 values (26, 32, 38, 46, 83) from actual used values used. (BTW: the 1% series values, E96, are correct!)

Can anyone shed light on this (mistake?). Did someone goof around the turn of the century?

---Jim

(*) For E12 series: k=10^(1/12)= 1.2115277
Values are then: k^n (n=0,…,11)

 

[advidana]

I think it had to do with color coating of the resistors.

 

[electricuwe]

Maybe for calculating the values the factor has 1,2115277 has been rounded or truncated to a lower number of significant digits. Remember, these standards were published at a time before scientific calculators were available to everyone.

 

[advidana]

electricuwe-I think you are right. They would use a side rule in those days.

 

[jims99]

Thanks electricuwe and advidana for your insights.

I tried calculating with coarser roundoffs and truncations, but this didn't yield better results to justify chosen values. For example, when I used 1.21 as k, things got worse as six values were off (instead of five). And you may note that the calculated 83 values is higher rather that lower like other off values.

While recognizing they may have used slide rules, the fact is that log tables (with at least 4 sig. digits) were available and widely used more than 100 years ago.

My guess is is someone didn't bother for better accuracy and used a slide rule sloppily to arrive at today's resistor values.

My hope is that another "historian" on this board will share more.

jim s.