This counter-electro-motive force, which develops while the machine is in motion, makes it unnecessary to hold back the current longer by the extra resistance of the rheostat and hence that is usually cut out. Being used only for starting purposes and looking like a box, it is generally called the "starting box." If now it was intended that this motor should run at a constant speed, as is often the case, no other governor would be needed than this counter-electro-motive force, for whenever the machine begins to go faster, on account of reduced load, its counter-electro-motive force increases as the speed and holds in check the impressed electro-motive force. This acts very perfectly as a governor, and motors operate with notoriously constant speed under variable loads. But, of course, in this present instance the motor is required to work at a variable speed. It must pump air slowly for the soft passages of music, and it must work the pump to its utmost for the very strong passages. [Illustration: Fig. 23] To understand how an electric motor may pump an organ and have its speed automatically controlled, let us examine the diagram in Fig. 23. The motor _m_ causes the shaft _S_ to revolve, carrying the crank _C_ around with it. The rod _r_ causes _a b_, the lower side of the bellows, to rise and fall, this side being hinged at _b_. The side _b c_, is fixed. When the side _a b_ is pushed upward by the crank rod the valve _f_ closes and the air in the compartment _h_ pushes open the valve _g_ and enters the compartment _j_. The upper side _d e_, of this compartment rises as it is filled with air. Weights _K_, _K_, _K_, rest on the top of this and air ducts lead from this compartment to the pipes of the organ. The keys of the organ operate air cocks which open and close the air ducts connected with the organ-pipes. A chain connected with _e_ passes around the axle of the wheel _l_ and has a weight _W_ upon its lower end. The wheel _l_ carries a strip of brass _n_, which slides over metal points _p_, _p_, _p_, etc. The successive points are connected by coils of wire to furnish resistance. This series of coils is called a rheostat. The wires _t_ and _u_ form a loop from the armature of the motor and connect this rheostat in series with the armature. _u_ is connected with the brass strip _n_. Notice that when the compartment _j_ is full of air and the side _d e_, is lifted to its greatest height the strip _n_ is moved to the lowest point _p_, and the electric current must pass from _u_ through all the resistance of the rheostat in order to get back to the armature by the wire _t_. This makes the motor go very slowly. When _d e_ sinks down, the strip _n_ moves to the upper points _p_, and the resistance is reduced step by step, enabling the motor to quicken its speed and pump faster as more air is required.