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 1/2 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 1/2 inch by drawing a stop-handle, the stickers _e_ will then be too short by 1/2 inch, or the pull-downs _h_ will be too long in an equal degree, and the manual will be silenced.
An easy way of making such a bridge rise and fall is shown in Fig. 56. The bridge _a_, which cannot be too solid and heavy, is held between guides, _c d_, which are blackleaded and accurately adjusted. At each end of the bridge is fitted a little roller or wheel of box-wood, turning freely on a pin. To the frame below is fitted another such roller, or two, as in the figure. Between these two or three rollers, at each end of the bridge, an inclined plane of wood, _e_, is made to traverse by means of a horizontal trundle and arms. It will be seen at a glance that when the inclined plane is pushed between the rollers by drawing or thrusting in a stop-handle, it lifts the bridge between the guides and dislocates the action; when it is withdrawn, the bridge resumes its place by its own weight, and the action is again in order.
It is now easy to understand the construction of the remaining couplers. The choir-coupler (Fig. 52, p. 152), has two bridges, _x_, _y_, with two sets of short backfalls and communicating stickers. If the bridges are fixed, then on depressing any key on the upper manual its tail raises the backfall, which presses down the lower backfall, which in its turn pushes up the tail of the lower key and causes the note to be heard. If the upper bridge be made to rise or the lower to fall 1/2 inch, then the keys are at once disconnected.