Why is the first band on a resistor never black?

Question

I'm taking an electronic circuit analysis class, and I was asked to give the color of the 3rd band of a 1MΩ resistor. My answer was blue, thinking it could be black-brown-blue (01 * 1MΩ), but the automated quiz said the correct answer was green (brown-black-green).

So I did some research, thinking there were just multiple correct answers, and I read in a few places that the first band on a resistor is never black (0). Why is this? Does a black first band have some other meaning? It would really help me remember it if I knew the history or reasoning behind it.

This question has been answered. For further reading on zero-ohm resistors mentioned in the answers and comments here, I found these questions and answers helpful:

1. What is the usage of Zero Ohm & MiliOhm Resistor?
2. Zero ohm resistor tolerance?

Answer 1

The first band is never black for the same reason that you always write numbers scientific notation with a single nonzero digit in front of the decimal place (e.g. 6.022e23) - convention. Generally the resistor specifications will all have the same number of nonzero significant digits (2 or 3, depending on the tolerance) except for a couple of values, namely even powers of ten (1, 10, 100, etc) and possibly a few others by coincidence.

Answer 2

Here is an example of how these codes would be read on a garden-variety 1-kilohm (or “1k”) resistor. Reading the bands from the left, the brown band represents 1, and the black band represents 0. Putting these together gives “10” for the base value. (The first band is almost never black, except in the unusual case of a “zero-ohm” resistor: a single black band, otherwise known as a “wire.”) The third band is the multiplier band; red signifies multiplication by 100, making the total value 10 x 100 = 1,000 ohms — or “1k.” The 4th band is the tolerance band; gold signifies that the true resistance value of this component should be within 5% of the stated value (950 to 1,050 ohms).

Answer 3

There are additional cases where the first band can be black besides the wire or jumper case. I have some .100 ohm resistors and some .050 ohm resistors that are coded with a black first band. Otherwise, how would you code them?

Blk, Brn, Blk, Ag, Brn = 010/100 1% = .10

Blk, Grn, Blk, Wht, Brn = 050/1000 1% = .050 +/- 500 uOhms

Probably whoever coded these decided the 10^9 multiplier was unneeded and made this up. After all, how often do you see a 100 Gohm resistor?

These probably came from Ohmite, as the picture on their site is identical to the resistors that I have, including the color code.

Answer 4

I believe this is a black first resistor. Reading the top (horizontal) resistor right-to-left. Not as obvious in this photo, but to the naked eye, the left (tolerance) band is definitely gold. The first band looks 100% black.

It's reading a consistent 148 Ω, but the left lead heated up and separated from the board. As I read it, it's black, Black or Brown, Orange, Gold. Damaged resistors (as I understand it) usually increase in resistance (though can short [usually high value resistors]). Black first essentially would be a 0 Ω resistor (so maybe a jumper at best). But it gives a consistent 148 Ω reading. Another of the exact same resistor in another electronic burnt up and reads irregularly (manipulating the leads changes value drastically). I've seen as low as 6 kΩ and as high as half a mega-ohm (so it's fried).