The Sliding Head can be Set so That Any Size Ellipse may be Drawn within the Scope of the Instrument
To operate the ellipsograph lay out the length of the major axis on a center line, then bisect the distance between these points and erect a perpendicular. On this line lay off half the minor axis, measuring from the center line; then from this point locate the foci by setting the dividers to half the major axis and scribing arcs of circles to cut the center line. Set the heads of the instrument so that the projecting needle ends will be the same distance apart as the foci, and clamp the sliding head with the wedge. Set the instrument so that the needle eyes will be exactly over the points where the foci are on the center line. A pencil, with a little groove filed ¹⁄₈ in. from the point, for the thread to run in, is set to half the minor axis and the thread is pulled taut and secured by the thumb tack. The pencil should be held perfectly perpendicular while scribing the line. The instrument must be reversed to draw the other half of the ellipse.
To draw an ellipse that will be an exact projection of a circle at any given angle it is necessary to determine the length of the major axis. This may be done by laying out the circle, either full size or to scale, and projecting two parallel lines equal to the diameter of the circle, or its scale, and connecting these lines with a line drawn to the required angle. The length of this line is the major axis.
¶Never run a glass cutter over the same line twice, as this will ruin the cutter. Alcohol rubbed along the line to be cut aids in the process.
Ship’s-Wheel Device for a Radiator Valve
Leaving one’s comfortable bed to open the valve of a radiator in a cold room is an unpleasant task. The device shown in the sketch obviates the necessity for leaving the bed, yet gives as positive control over the valve as if the hand were on the valve wheel.
Draw on the Proper Cord to Open or Close the Valve
The construction of the pulley attached to the top of the valve wheel is shown in the small sketch at the right. It is built up of a center section of wood and flanges of sheet metal, fastened with screws. The pulleys attached to the wall are built in the same way, but are smaller. Any size that is convenient may be used for the larger as well as the smaller pulleys, but the larger pulley must be small enough so that it will not rub against the end of the radiator.
The cord is wound around the pulley at the valve handle, several times, like the steering cord on a motorboat. This gives a positive grip on the pulley. The cord may be conducted directly from the large pulley to the nearer small pulley, the other small one being omitted.