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Silk-Thread Hygrometer with a Magnifying Sag and Pointer (Boy Mechanic, 1913)
A humidity indicator from a silk thread stretched with a slight sag between two nails: as dry air shrinks the silk, the sag straightens and pulls a second thread wound round a pencil-sized axle, swinging a pointer over a scale.
Beginner
An hour
Instructions
1
1
Why the sag magnifies
Why the sag magnifies
Loading Jupyter Notebook...
2
2
Read the Boy Mechanic
Read the Boy Mechanic
"A very small shrinkage of B, such as occurs when the atmosphere is dry, will cause an increased movement of C."
3
3
String thread B
String thread B
Drive a nail at each end of a board A and stretch silk thread B between them, not quite straight: let it bend a little toward the pointer bracket D.
Materials for this step:
Pine Board1 piece
Silk Thread1 piece
Nails1 pieceTools needed:
Hammer4
4
Make the pointer axle
Make the pointer axle
Cut a round wooden axle about the size of a lead pencil and drive a pin into each end. Cut a piece of tin E V-shaped at each end and bend the ends up as bearings for the pins; fix it on bracket D. Fix a light pointer on the axle.
Materials for this step:
Hardwood Dowels1 piece
Dressmaker's Pins1 piece
Tinplate Sheet1 pieceTools needed:
Tin Snips
Flat-Nose Pliers
Hand Saw5
5
Connect thread C
Connect thread C
Tie a second silk thread C to the middle of B, fasten it to the axle and wrap it once or twice round, so pulling C turns the pointer over a scale card.
Materials for this step:
Silk Thread1 piece
Cardboard Template Sheets1 piece6
6
Read it
Read it
When the air dries, B shrinks and straightens, pulling C and swinging the pointer; damp air lets it back. Mark the scale against a known hygrometer if you want numbers.
7
7
Troubleshooting
Troubleshooting
Hygrometer troubleshooting.
Flow
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8
8
History and honest limits
History and honest limits
From **The Boy Mechanic, Book 1** (Popular Mechanics, 1913), pp. 71-72 (checked on the archive.org scan boymechanic0000unse_n5a8). He gives no sizes or calibration; ours are marked.
Materials
7- 1 piecePlaceholder
- 2 piecesPlaceholder
- 1 piecePlaceholder
- 1 piecePlaceholder
- 1 piecePlaceholder
- 1 piecePlaceholder
- 1 piecePlaceholder
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