Microgram Balance – Jon Wallace

(adapted from an article in Scientific American (The Amateur Scientist, June 1996, pg.100)).

Background: During this last year, Sue Fennelly and I worked on a project concerning flight. Our goal was to design a lab that used ‘real world’ science and taught some of the basic concepts of flight. We decided to use insects to make a graph of wing loading and area. The only problem was that we had to determine the mass of these insects and many were too small for a conventional scale. I happened to find the article listed above and found that it worked quite well.

Building Tips: (Basic building directions are given in the article - see schematic below for potentiometer changes). Unfortunately, the original plans didn’t work properly for the galvanometer I purchased so I had to ‘play’ with the potentiometers. After trying several combinations I determined that a 500 ohm, 10 turn potentiometer worked best for the scale voltage and a 100 Kohm, 10 turn potentiometer for the offset voltage worked best with my galvanometer. [I bought a –35-0-35 mA Galvanometer from Carolina Biological Supply Company (#BA-75-7290 - about $14) and a milligram mass set (#BA-70-2450 – about $12)]. The balance uses a galvanometer coil with a voltage across it to determine the force needed to lift the object on the small aluminum foil pan. A voltmeter is used to measure the voltage applied across the coil. The unit must be calibrated each time you use it and needs to be re-calibrated if it sits for more than a few minutes for best accuracy. The usable range seems to be about 1 mg to 400 mg. I have used the scale to measure the mass of ‘chilled’ flying insects and found the values I got to agree very closely with published values. I have also done a few graphs of the calibration measurements and all are incredibly close to a linear response and therefore remarkably accurate for such an inexpensive device. (see graph) .