Why are resistor values spaced irregularly?

Question

When in electronics labs I always see resistors with values like 47, 52, 58, 1.2k. Why are they not linear and nice, why choose such strange values?

Answer 1

They are spaced to cover the range of resistance for a given tolerance.

For example, if the tolerance for a set of resistors is +/-10%, then the base values will be:
1
1.2
1.5
1.8
2.2
2.7
3.3
3.9
4.7
5.6
6.8
8.2
9.1

1 covers the range between 0.9 and 1.1
1.2 covers the range between 1.08 and 1.32
1.5 covers the range between 1.35 and 1.65
1.8 covers the range between 1.62 and 1.98
2.2 covers the range between 1.98 and 2.42
and so on...

Everything is "covered"

For higher tolerance resistors, the ranges get smaller so there are more values needed to cover the gaps.

There are standard values for each tolerance set: 12 values (listed above + "1" always a "1' in the mix) for 10% resistors, 24 values for 2% to 5% resistors, 96 values for 1% resistors, and 192 values for 0.1% to 0.5% resistors. Anything more precise than that and you start to pay more money for custom valued resistors.

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Answer 2

Some values just pop up over and over in normal electrical equations. Such as 50 ohm impedance, or resistance, or 250 ohms when dealing with 4-20 ma signals and 0-10 volt signals. I think, these values you mentioned above are most common and that is why are are manufactured based on need.