The Shutter.—­The woodwork is so simple that nothing further need be said about it.  The more difficult part of the business is the making of the shutter, which must be so constructed that it can be opened and closed rapidly by motions similar to those used in working the telegraph key described in a preceding chapter.  Speed of working is obtained by dividing the shutter into two or three parts, each revolving on its own spindle, but all connected so as to act in perfect unison.  The thinnest sheet brass or iron obtainable should be used, so that the tension of the spring used to close the shutter need not be great.  Our illustration shows a two-part shutter, each half an inch wider than the hole in the front, and jointly a similar amount deeper.  The upper half overlaps the lower, outside, by a quarter of an inch.

The spindles are two straight pieces of brass wire, revolving in sockets which are most easily made of notched pieces of wood (as shown in Fig. 189), with removable caps of strip tin.  The lower spindle should be an inch longer than the width of the front, to allow for a cranked end, to which the closing spring will be attached.

Having cut out the halves of the shutter, solder the spindle wires to one edge of each on what will be the back side.  The wires must be so arranged as to allow a quarter of an inch to project beyond the left edge of the front, as the opening mechanism is situated on this side as the most convenient for the operator.

Take a couple of metal discs, an inch or so in diameter, and bore a hole in each near the circumference to fit the ends of the pivots fairly tight.  Three-eighths of an inch from this—­centre to centre—­bore and tap a hole for a small screw.  The tapping should be done with a taper tap and carried just so far that the screw turns stiffly without danger of being broken off by the screw-driver.

Next find the correct positions of the parts of the shutter and the spindle sockets on the front of the holder, and mark them off carefully.  Screw the wooden parts of the sockets to the front.  Four little “distance pieces” should now be cut out of small tubing, or made by twisting tin round the spindle, to place on the spindles between shutter and sockets, so that the shutters cannot shift sideways.

The right-hand end of the lower spindle must be bent over (after slipping on the distance piece) to form a 1/2-inch crank making an angle of 45 degrees with the line of the front, in an upward direction, as it will be depressed by the opening of the shutter.  Flatten out the end with a hammer, and drill a small hole near the tip.

The shutters can now be placed in position, and the caps of the sockets be screwed on.  The next thing to make is the connecting rod to join the cranks at the left side of the front.  For this purpose we may use a piece of fairly stiff strip metal—­brass by preference—­5 or 6 inches long.  Half an inch from one end make a mark with the centre punch; then measure off exactly the distance between the shutter spindles, and make a second punch mark.  Drill holes at the marks large enough, for the disc screws to pass through easily, but not loosely.