WHEN much hatching is to be done there is always a tendency to move the T-square out of position because it is necessary to hold the T-square and at the same time to manipulate the triangle and the ruling pen. To eliminate this trouble the writer took two small electro-magnets out of an old door bell, and fastened one on each end of the head of the T-square, taking care to get the magnet cores on a line with the edge of the head. Then a piece of thin iron band was fastened to the edge of the drawing board with several small screws. The magnets were connected in series with a battery and switch. When turning on the current, the magnets will hold the T-square firmly against the drawing board. Then all one needs to do is to attend to the triangle and the pen. This will eliminate slipping of the T-squareand make for accurate work.
This article was originally published with the title "Magnetic T-Square" in Scientific American 105, 15, 317 (October 1911)