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builDing a harmonograPh builDing a harmonograPh Pen Lever Table Clamp Gimbals Clamps can be obtained from suppliers of laboratory equipment. For some of the drawings top weights are needed, held in place by clamps. Clamps can also be added to pendulum tops for fine tuning, with one or more metal washers added. The simpler kind of bearing consists of brass strips bolted into a slot in the pendulum and filed to sharp edges to rest in grooves each side. In a bearing involving less friction the pendulum is encased at the fulcrum in a horizontal block of hardwood with vertical bolts each side filed to sharp points and resting in dints in metal plates. If drilling the large hole in the block is too difficult, it can be made in two halves, each hollowed out to take the shaft and bolted together. Rotary motion needs gimbals. Here the grooves for the pendulum are filed in the upper side of a ring e.g. a keyring while the under side has grooves at rightangles to the upper ones. The lower grooves fit on two projecting sharp edges brass strips, each enclosed between two pieces of wood fixed to the table. With the alternative bearing a large flat washer should be used with depressions to take the sharp points. Pen arms should be as light as possible to minimise tophamper. They are easily made from balsawood strips sold by modelmaking shops, using balsa cement and scotch tape. For two pendulums the arm can be fastened to the shaft with pinchedoff needles, and the pen jammed into a hole at the other end. For three pendulums the side pieces on the arm should enclose its shaft firmly but not too tightly and be held gently with a thin rubber band. One of the arms holds the pen, while the other is held by protruding needles pushed in backwards and secured gently at both ends by the rubber bands. There may well be better, more sophisticated ways of doing all this. All suggestions welcomed. An additional fitting is needed to lock a rotary pendulum so that the instrument can be used with ANyONE seriously interested in making a harmonograph should consider going straight for the threependulum model. The table must be highly rigid and firm on the floor, otherwise the movements of the weights will be distorted. I suggest it should be about 90 cm above the floor, 60 x 30 cm for two pendulums, 60 x 60 cm for three, and some 2 cm thick with an apron all round, about 8 cm deep. Legs should be about 6 cm square, splayed outwards and pointed at the bottom. One way of achieving the splay is to fix wood or metal brackets in the corners under the table each side of the diagonals and bolt the legs between them. After adjusting the legs to give the correct splay they can then be fixed in position with screws through the apron. To save space, slice off the table as along the dotted line. Three legs are not quite so stable, but work fairly well. The platform carrying the paper should be light and rigid, fixed to the pendulum with a countersunk screw. A size of 22 x 16 cm will conveniently take half an A4 sheet secured by a rubber band or small clip. All sizes suggested are maxima, but a scaled down version will still work if it is carefully made. If you are tempted to make a harmonograph, start with the weights, for the instrument will only be satisfactory if these are really heavy and yet easy to adjust. It is a good idea to make about ten of, say, two kilos each, so the loadings can be varied. They should be about 8cm in diameter, with a central hole, or better with a slot for easier handling. Either cast them yourself from lead or readymixed cement or have them made by a metal smallware dealer or friendly plumber. The shafts should be made from wood dowel, about 1.5 cm in diameter metal rods are liable to
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