A small portable harmonograph will be found to be a good means of entertaining friends at home or elsewhere. The gradual growth of the figure, as the card moves to and fro under the pen, will arouse the interest of the least scientifically inclined person; in fact, the trouble is rather to persuade spectators that they have had enough than to attract their attention. The cards on which designs have been drawn are in great request, so that the pleasure of the entertainment does not end with the mere exhibition. An album filled with picked designs, showing different harmonies and executed in inks of various colours, is a formidable rival to the choicest results of the amateur photographer's skill.
Fig. 169. Goold's Twin Elliptic Pendulum Hamonograph.
Fig. 170. Benham's miniature Twin Elliptic Pendulum Harmonograph.
For the Rectilinear type of harmonograph wooden rods 5/8 to 3/4 inch in diameter will be found very suitable. They cost about 2d. each. Be careful to select straight specimens. The upper pendulum of the Miniature Twin Elliptic type should be of stouter stuff, say a broomstick; that of the Goold apparatus stouter still.
All pendulums on which weights are slid up and down should be graduated in inches and fractions, reckoning from the point of suspension as zero. The graduation makes it easy to re-establish any harmony after the weights have been shifted.
For a harmonograph to give satisfaction it is necessary that very little friction should be set up at the point of suspension, so that the pendulums may lose amplitude of swing very slowly.
One-way suspensions are easily made. Two types, the point and knife-edge respectively, are shown in Fig. 168 and the top part of Fig. 172. The point suspension is most suitable for small rods and moderate weights; the knife-edge for large rods and heavy weights which would tend to crush a fine point.
Points should rest in cup-shaped depressions in a metal plate; knife-edges in V -shaped grooves in a metal ring.
Screws turned or filed to a sharp end make convenient points, as they can be quickly adjusted so that a line joining the points lies exactly at right angles to the pendulum. The cups to take the points should not be drilled until the points have been thus adjusted. Make a punch mark on the bedplate, and using this as centre for one of the points, describe an arc of a circle with the other. This will give the exact centre for the other cup. It is evident that if points and cup centres do not coincide exactly there must be a certain amount of jamming and consequent friction.
In making a knife-edge, such as that shown in Fig. 172, put the finishing touches on with a flat file drawn lengthwise to ensure the edge being rectilinear. For the same reason the V slots in the ring support should be worked out together. If they are formed separately, the chances are against their being in line with one another.
Gimbals, or universal joints, giving motion in all directions, require the employment of a ring which supports one pair of edges or points (Fig. 172), and is itself supported on another pair of edges or points set at right angles to the first. The cups or nicks in the ring should come halfway through, so that all four points of suspension shall be in the same plane. If they are not, the pendulum will not have the same swing-period in all directions. If a gimbal does not work with equal freedom in all ways, there will be a tendency for the pendulum to lose motion in the direction in which most friction occurs.
By wedging up the ring of a gimbal the motion of the pendulum is changed from universal to rectilinear. If you are making a harmonograph of the type shown in Fig. 168, use a gimbal for the platform pendulum, and design it so that the upper suspension gives a motion at right angles to the pen pendulum. The use of two little wedges will then convert the apparatus in a moment from semirectilinear to purely rectilinear.
The provision of weights which can be slipped up and down a rod may present some difficulty. Of iron and lead, lead is the more convenient material, as occupying less space, weight for weight, and being more easily cast or shaped. I have found thin sheet roofing lead, running 2 lbs. to the square foot, very suitable for making weights, by rolling a carefully squared strip of the material round the rod on which it will have to move, or round a piece of brass tubing which fits the rod. When the weight has been rolled, drill four holes in it, on opposite sides near the ends, to take nails, shortened so that they just penetrate all the laps but do not enter the central circular space. These will prevent the laps sliding over one another endways. A few turns of wire round the weight over the heads makes everything snug.
Just one caution here. The outside lap of lead should finish at the point on the circumference where the first lap began, for the weight to be approximately symmetrical about the centre.
An alternative method is to melt up scrap lead and cast weights in tins or flowerpots sunk in sand, using an accurately centred stick as the core. This stick should be very slightly larger than the pendulum rod, to allow for the charring away of the outside by the molten metal. (Caution.--The mould must be quite dry.)
Failing lead, tin canisters filled with metal scrap may be made to serve. It will in this case be necessary to bore the lid and bottom centrally and solder in a tube fitting the rod, and to make an opening through which the weighting material can be inserted.