(a) The common Tumbler coupler is represented in Fig. 53. a is a slender bridge, having as many notches as keys in the manual, and fitted with short stickers called tumblers. These tumblers, or stickers, are generally rectangular in section, and they must fit the notches neatly but with freedom of motion; the notches are closed in by a tringle of wood glued over them, thus forming a series of mortices; and each sticker has a little pin, or a pair of pins, to prevent it from falling out of its notch or mortice. Or, the tumblers may be made from round rods, and may pass through round holes in the bridge. They are well blackleaded in either case. Now if this bridge, with its tumblers, is placed between the manuals near their hinder extremities, if the length of the tumblers is equal to the interval between the upper and lower key-tails, it is evident that on pressing down a key of the lower manual the tumbler will push up the tail of the corresponding key on the upper manual, and so on throughout the full compass of both. To reverse this, and to leave the manuals separate and independent as before, the bridge is made to take a quarter of a revolution in sockets fitted to carry its ends, which are rounded or turned in the lathe; the tumblers, by this partial revolution, are then no longer perpendicular, but parallel to the key-tails, as shown by the dotted lines in the figure, and cease to be touched by them. On the whole, this is the simplest form of swell-coupler.
Fig. 54.
The tumblers may also be placed on or off by causing the bridge to slide between guides for a distance of 1 inch or 2 inches. When the bridge is pushed back by drawing a stop-handle (which acts upon a horizontal trundle with arms and traces to give the sliding motion), the tumblers act on the key-tails; when it is drawn forward by thrusting in the stop, they fall into a hollow cut in the key-tails, and are too short to be of use. The hollow in the key-tails is bevelled, and the inclined plane so formed is leathered and blackleaded (see Fig. 54).
Fig. 55.
The ram coupler, Fig. 55, acts in a way closely resembling the last. Instead of tumblers, the bridge carries a set of short backfalls, turning on a wire as usual, and cut at the free end into a circular form. The sliding of the bridge brings these circular ends into contact with the key-tails of both manuals, or places them in a hollow cut in the keys, bevelled, leathered, and blackleaded as before. The ram-coupler can be used between manuals arranged too closely to admit of tumblers.
(b) It will facilitate our description of the choir-coupler and pedal-couplers if we here point out that if a bridge with backfalls (or squares) be made to rise or fall ½ inch or more at pleasure, the action connected with it will be thrown into or out of gear at the will of the player. If, in Fig. 32, for instance, the bridge g be made to rise ½ inch by drawing a stop-handle, the stickers e will then be too short by ½ inch, or the pull-downs h will be too long in an equal degree, and the manual will be silenced.
Fig. 56.