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.

[Illustration:  Fig. 175.—­End of pen lever.]

Attaching Pen to Lever.—­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.

[Illustration:  Fig. 176.—­Clip to hold glass pen.]

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.

Making Pens.—­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.