Photo Credit: YouTube Screenshot
By Andrew Bennett
The Cavendish gravity experiment requires us to be able to measure how much a hanging bar rotates when we place heavy weights nearby. In order to determine a reliable value for G, the gravitational constant, we must first measure the shift angle very precisely.
In this video, I build a protractor with a 3-meter radius that can measure angular shifts of as little as one one-thousandth of a radian. That's less than 0.06 degrees or about 3.5 minutes of arc!
Before I put the pressed wood into the groove, I had to add the markings. Since we know that an angle of 1 radian within the circle will give a distance along the edge of the circle equal to the radius, I simply marked every 3 meters along the wall material to serve as the whole radian marks. The tenths were marked out every 0.3 meters (30 cm), the hundredths every 0.03 meters (3 cm), and finally, the thousandths were marked every 0.003 meters (3 mm).
With the markings done (and my hand nearly crippled from the thousands of tiny marks I measured and drew), the only step left was to slide the walls into the groove. If you're interested in seeing more of the design or build process (time-lapsed, don't worry), check out the video below!
Although my giant protractor is for the Cavendish experiment, I'd recommend everybody keep one around the house in case of an angle measurement emergency. It's just good planning!
And if you missed a previous post in this series, click here to see what you've missed.
In this video, I build a protractor with a 3-meter radius that can measure angular shifts of as little as one one-thousandth of a radian. That's less than 0.06 degrees or about 3.5 minutes of arc!
How I Built My Protractor
The design is pretty straightforward but is extremely labor-intensive to construct one this size. I start by making a circular wall to use as the surface of the protractor made of thin, white, flexible pressed wood. I glued some pieces of 2x4 to my garage floor and used a router to cut a groove that the wall will fit into. To make the groove along a circle, I first traced out the circle on the wood by attaching a marker to a long wire, which then got attached to the center of the desired circle.Before I put the pressed wood into the groove, I had to add the markings. Since we know that an angle of 1 radian within the circle will give a distance along the edge of the circle equal to the radius, I simply marked every 3 meters along the wall material to serve as the whole radian marks. The tenths were marked out every 0.3 meters (30 cm), the hundredths every 0.03 meters (3 cm), and finally, the thousandths were marked every 0.003 meters (3 mm).
With the markings done (and my hand nearly crippled from the thousands of tiny marks I measured and drew), the only step left was to slide the walls into the groove. If you're interested in seeing more of the design or build process (time-lapsed, don't worry), check out the video below!
If you're reading via email, click here to view the video.
Although my giant protractor is for the Cavendish experiment, I'd recommend everybody keep one around the house in case of an angle measurement emergency. It's just good planning!
What's Up Next in the Cavendish Experiment Video Series?
In the next video, I'll cover the frame for the experiment in plastic, place a mirror and laser to allow us to measure the angle, and get the torsion bar set up for measurements. We're getting so close to the end on this! Be sure to subscribe, so you don't miss these last few videos.And if you missed a previous post in this series, click here to see what you've missed.
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