A Defective Ruler

You might ask: who would actually want a deliberately defective ruler? On one hand, it is a great gift for people who aren't particularly observant or like a good gag. 

Otherwise, I intended this to be a teaching tool in a science classroom, showing students not to implicitly trust their measuring devices and to ensure they are actually accurate and precise first. You might hand these out for them to use in some practical that requires measuring lengths, then point out the systematic error later [or let them discover it ;)].

There are three main defects in the ruler.

1. The 8 cm mark is missing, meaning it should really only go up to 14 cm. 
2. Even then, the true length of the ruler is 130 mm not 140 mm, since it was scaled down by 10 mm.
3. The edge with the markings is not straight, it curves ever so slightly outwards.

With knowledge of the first 2 defects you should be able to get back the true lengths from the bogus measurements this ruler provides. For instance, if you use the defective ruler to measure a length of 12 cm, you would need to:

1. Subtract 1 cm since the 8 cm mark is missing. 
2. Multiply by (13/14) to scale down the measurement accordingly.

This converts the supposed length of 12cm to the true length of ~10.2 cm, which is close enough to the true value given the 2 mm thickness of each major marker. Be sure to skip the first step if the length you measure is less than 9cm.