Make a Balance Scale accurate to 1/2 gram, using a ruler and 2 old CDs

Picture of Make a Balance Scale accurate to 1/2 gram, using a ruler and 2 old CDs
This is my first instructable.  The reason I made this scale is that I am a photographer and I mix my own developers using household compounds like instant coffee ( Caffinol---a future instructable) and I need to measure quantities as small as .5 grams.  The electronic kitchen scales are not accurate down to that level, so I decided to see if I could make a balance scale that would do the job.
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Step 1: Materials and Tools

Picture of Materials and Tools
You will need a plastic ruler, 2 old scratched CDs (new would do but why waste them),  6  6" fishing leaders with snap ends,  2 small pieces of plywood,  long finish nails and some wood screws.  You will also need a drill with an assortment of bits, a pencil and a US Nickel.

Step 2: The Ruler:

Picture of The Ruler:
The ruler is the beam for the scale and must be as symmetric as possible.  The ruler I used was a plastic Wescott with a raised thumb hold ridge down the length of it.  It has a hole at one end to hang on a hook.  Pick a drill bit of the same diameter and drill an identical hole at the opposite end.  then drill a small hole next to both of the large holes. The small holes are for hanging the discs.    Next pick a drill bit that is slightly larger than the finish nail, find the mid point on the ruler, on this ruler it is right at the 6 inch mark.  Drill a hole through the raised holder, as close to the flat surface of the ruler as possible.

Step 3: The CDs

Picture of The CDs
scale 1.jpg
Stack both CDs evenly and drill 3 evenly spaced holes near the edge of the discs and clip 3 fishing leaders to each disc.  Take two paper clips and thread  each set of leaders onto them ( I did not have any paper clips so I used a finish nail bent into the shape of an elongated "C") and hang them on from the holes at the ends of the ruler.