satisfactory still is a pump with three cylinders and cranks set 120 degrees apart. With this arrangement the flow of water is so uniform that the use of an air-vessel as an equalising agent becomes almost superfluous. The two-cylinder pump is the mechanical analogy to the two-phase motor, and the three-cylinder pump is that of the three-phase motor.
Since, as will be shown in the next chapter, the use of three phases has also the advantage of considerable economy in the amount of metal required to carry a given power over a given distance, the use of three-phase current for motive power purposes has become almost universal,
Small asynchronous motors are sometimes made with what is technically known as a "squirrel cage rotor," the name being derived from the peculiar type of winding used. In the ordinary sense of the word the conductors on the rotor do not form a winding of wire, but a series of copper bars laid along and embedded into the surface of the core. At either end the bars are all joined up by metal rings, thus forming a kind of squirrel cage. This construction has the advantage of great mechanical simplicity and strength, but the disadvantage that at starting the motor takes
