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. [Illustration: Fig. 24] The organist before beginning to play closes a double-pole, single-throw switch (Fig. 24), which sends the electric current to the motor. The motor pumps air until the bellows is full, and if the organist delays playing, the strip of brass _n_ (Fig. 23) is carried below the lowest point _p_, thus cutting off the current and stopping the motor. As soon as he uses some of the air in the bellows, however, _n_ rises and makes contact with the points _p_ and the motor starts. This suggests that a somewhat similar thing is accomplished under electric cars which have air brakes. An electric motor pumps the air and compresses it in a tank. When the pressure reaches a certain point, say sixty pounds per square inch, it automatically shuts off the electric current from the motor which works the pump. But when the motorman uses some of the air to apply the brakes to the wheels, and the pressure in the tank falls below sixty pounds, the electric current is again automatically turned on to the motor.