Watt's Parallel Linkage

James Watt invented this linkage to transform rotary motion to an approximate a straight line motion. Of all his inventions, he was said to be most proud of this one.

Steps

1. Cut out the pieces from card stock. You can use an old manila folder if you don't have card stock. The short dotted lines at the top and bottom of the page can help you line up the cuts on a paper cutter.
   
2. Punch the holes where marked. You may have to cut out a hole with a knife if your hole-punch doesn't reach.
   
3. Use a knife to cut along the lines of the tabs.
   
4. Connect the two longest pieces to the two rectangles at the points A and B. To do this, lift up the two tabs perpendicular to the piece and curl the ends in so that the pieces with the holes can slip over them. Flatten the tabs again, thus holding the pieces in place.
   
5. Lift up the tabs at point C of the small crossbar and push the tabs at point D down. Connect it to the longest pieces. Note that the tabs at D will go down to the underside of the model.
   
6. Tape the rectangular pieces onto the table so that the two long bars are parallel with the short bar perpendicular to both. Swing the long arms and notice that the hole in the short bar travels in a nearly straight line for part of its path.
   

References

• (Kempe 1877)