Using a set square to reduce parallax error

[MBBphys]

Homework Statement

In one of my revision guides, there is an experiment for calculating the acceleration of free fall using the electromagnet and trap door arrangement. The biggest error is in the measurement of the height over which the ball falls. The guide says: "You could reduce the error in your measurement of h by using a set square to make sure your eye is level with the ruler".
What does that mean? How do you do that?
Thanks!

Homework Equations

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The Attempt at a Solution

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[collinsmark]

Maybe this?

https://en.wikipedia.org/wiki/Set_square

By the way, I'm not familiar with this experiment using an electromagnet and trap door, the way you described it. So I'm not sure how the "set square" fits in. You might want to be more specific about the experimental details; perhaps post the entire question verbatim.

 

[CWatters]

Parallax error occurs when the line between your eye the object and the ruler isn't 90 degrees to the ruler.

I would normally insert a diagram to explain but unfortunately I'm stuck with this tablet for a few days.