As lead is too soft a metal to give a satisfactory purchase to a screw--a thread cut in it soon wears out--it is better to support a leaden weight from underneath by means of a brass collar and screw. A collar is easily made out of a bit of tubing thickened at the point where the screw will pass by soldering on a suitably shaped piece of metal. Drill through the reinforcement and tubing and tap to suit the screw used, which may well be a camera tail screw, with a large flat head.
I experienced some trouble from the crushing of wooden rods by a screw, but got over it as follows. The tubing selected for the collar was large enough to allow a piece of slightly smaller tubing to be introduced between it and the rod. This inner piece was slit from one end almost to the other, on opposite sides, and soldered at one end to the outer tube, a line joining the slots being at right angles to the axis of the screw. The pressure of the screw point was thus distributed over a sufficient area of the wood to prevent indentation. (See Fig. 173.)
Fig. 171. Gimbal giving universal movement: point suspension.
The pen lever, of whatever kind it be, must work on its pivots with very little friction, and be capable of fine adjustment as regards balance. For the Rectilinear Harmonograph the form of lever pivot shown in Fig. 174 is very suitable. The spindle is a wire nail or piece of knitting needle sharpened at both ends; the bearings, two screws filed flat at the ends and notched with a drill.
Fig. 172. Knife-edge universal-motion gimbal.
The brass standard should be drilled and tapped to fit the screws fairly tight, so that when once adjusted they may not slacken off. If the lever is made of wood, the tail may be provided with a number of metal pegs on which to place the weights; if of wire, the tail should be threaded so that a brass weight and lock screw may be moved along it to any desired position. It is very important that the pressure of the pen on the card should be reduced to a minimum by proper balancing, as the friction generated by a "heavy" pen slows the pendulum very quickly; and that the centre of gravity should be below the point of suspension, to put the pen in stable equilibrium. The lever shown in Fig. 169 is suitable for the Twin Elliptic Pendulum.
In this case the lever is not moved about as a whole. Mr. C. E. Benham advocates the use of wood covered with velvet to rest the lever points on.
For keeping the pen, when not in use, off the platform, a small weight attached to the lever by a thread is convenient. When the pen is working, the weight is raised to slacken the thread.
In the case of wooden levers, it is sufficient to slit the end centrally for a few inches after drilling a hole rather smaller than the pen, at a point which lies over the centre of the card platform, and quite squarely to the lever in all directions, so that the pen point may rest squarely on the card. (Fig. 175.)
Another method is to attach to the end of the lever a vertical half-tube of tin, against which the pen is pressed by small rubber bands; but even more convenient is a small spring clip shaped as in Fig. 176.
The card platform should be perfectly flat. This is essential for the production of good diagrams. If wood is used, it is advisable to glue two thin pieces together under pressure, with the grain of one running at right angles to the other, to prevent warping.
Another important point is to have the card platform square to the rod. If a piece of tubing fitting the rod is turned up true in the lathe and soldered to a disc screwed to the underside of the table, perpendicularity will be assured, and incidentally the table is rendered detachable.
To hold the card in place on the table, slit a spring of an old photographic printing frame down the middle, and screw the two halves, convex side upwards, by one end near two opposite corners of the platform. (See Fig. 170.) If cards of the same size are always used, the table should be marked to assist adjustment.
The most satisfactory form of pen is undoubtedly a piece of glass tubing drawn out to a point, which is ground down quite smooth. The making of such pens is rather a tedious business, but if care be taken of the pen when made it will last an indefinite time.
Fig. 174. Pivot for pen lever.
Tubing 3/16 or 1/8 inch in external diameter is suitable. Break it up (by nicking with a file) into 9-inch lengths. Take a piece and hold its centre in the flame of a small spirit lamp, and revolve it till it softens. Then draw the glass out in as straight a line as possible, so that the points may be central. If the drawing is done too fast, the points will be much too long to be of any use: half an inch of taper is quite enough.
Assuming that a point of satisfactory shape has been attained--and one must expect some failures before this happens--the pen may be placed in the pen lever and ground down on a perfectly clean wet hone laid on the card platform, which should be given a circular movement. Weight the lever so as to put a fair pressure on the point.
The point should be examined from time to time under a strong magnifying-glass, and tested by blowing through it into a glass of water. For very liquid ink the hole should be as small as you can possibly get it; thick inks, such as Indian, require coarser pens.
The sharp edge is taken off and the width of the point reduced by drawing the pen at an angle along the stone, revolving it all the time. The nearer to the hole you can wear the glass away the finer will be the line made by the pen.
Another method is as follows:--Seal the point by holding it a moment in the flame. A tiny bulb forms on the end, and this has to be ground away till the central hole is reached. This is ascertained by the water test, or by holding the pen point upwards, so that light is reflected from the tip, and examining it under the magnifier. Then grind the edge off, as in the first case.
The ink should be well strained, to remove the smallest particles of "suspended matter," and be kept corked. Fill the pen by suction. On no account allow the ink to dry in the pen. Squirt any ink out of it when it is done with, and place it point downwards in a vessel of water, which should have a soft rubber pad at the bottom, and be kept covered to exclude dust. Or the pen may be cleaned out with water and slipped into a holder made by rolling up a piece of corrugated packing-paper. If the point gets stopped up, stand the pen in nitric or sulphuric acid, which will probably dissolve the obstruction; and afterwards wash it out.
Fig. 175. End of pen lever.
I have found Stephens's coloured inks very satisfactory, and can recommend them.
The paper or cards used to draw the figures on should not have a coated surface, as the coating tends to clog the pen. The cheapest suitable material is hot pressed paper, a few penny-worths of which will suffice for many designs. Plain white cards with a good surface can be bought for from 8s. to 10s. per thousand.
Fig. 176. Clip to hold glass pen.
Moisten one side of a clean lantern slide plate with paraffin and hold it over a candle flame till it is a dead black all over. Very fine tracings can be obtained on the smoked surface if a fine steel point is substituted for the glass pen. The design should be protected by a cover-glass attached to it by a binding strip round the edges.