Fig. 22
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.
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.
Small motors in order to be effective must travel at high speed. This motor when moving at its highest speed makes 1,800 revolutions per minute. The bellows on the other hand needs to be large and move slowly in order to be efficient. Hence the motor is not in reality connected directly to the shaft S, but causes the shaft to revolve by means of a series of pulleys and belts. The pulley on the motor is three inches in diameter. It is connected by a flat leather belt with a wheel thirty inches in diameter. When the motor therefore, makes 1,800 revolutions per minute this wheel makes 180 revolutions per minute. The axle of this wheel carries a small cog-wheel three inches in diameter and it is connected by a chain belt with a cog wheel on the shaft S ([Fig. 23]). Thus this shaft revolves thirty times per minute, that is, the rod r rises and falls each second. A pull of one pound on the rim of the motor pulley will cause a pull of sixty pounds on the cogs of the wheel upon the shaft S. If the second belt were leather, a sixty-pound pull would cause it to slip on the smaller pulley. Hence the second belt is a steel chain and the wheels have cogs, or sprockets, like a bicycle.
Fig. 24