Resist-O-Guide

I came across this “Resist-O-Guide” a while back and couldn’t resist (no pun intended) including it here, even though it is a little out of context.

For those not into electronics, resistors are little round things with wire pigtail leads out of each end. They have a series of bands on them to denote the resistance in ohms and the tolerance (acceptable error) from the stated value. They come in standard (called ‘preferred’) values such as 330 ohms, 1.2K ohms and 1.5M ohms, where the K is Kilo (1000) and the M is Meg (1,000,000). Each value 0-9 has a colour:

Black

0

Brown

1

Red

2

Orange

3

Yellow

4

Green

5

Blue

6

Violet

7

Grey

8

White

9

The first band is the first digit and the second band is the second digit, but the third band is the multiplier, taken as 10 to the power - so black (0) is 10 to the power 0, which is one, while brown is 1 and hence as a multiplier is 10. Brown Black Yellow becomes 10 with a multiplier of 4, for 100,000 ohms or usually called 100Kohms. The 4th band would be the tolerance, but I’ve never needed to use that - it’s easier to use a multimeter to measure the resistance of a selection of resistors of the specified value until you find the correct value, if it matters that much.

Looking at the card above, which is the front, you can see the values in the small holes in the middle. They are on circular cards that turn (you can just see the ‘dials’ at the top of the card). The corresponding colour is shown in the ‘resistor’ at the top of the card. In the one shown, the values are 1-0-000, or 10Kohms, which is brown-black-orange. Most resistors these days are 5% or less tolerance, but when this card was in use resistors were much more variable because of the method of manufacture, hence the specified tolerance.