There are three important methods of holding and rotating work in engine lathes, which may be referred to as turning between centers, chuck work, and faceplate work. In turning between centers, the work is supported by the 60° conical points of the live and dead centers. It turns together with the live centre on the dead centre. In chuck or faceplate work, the work to be machined is held in a chuck or a faceplate.
HYDROELECTRIC POWER-STATION
Water power was used to drive machinery long before Polzunov and James Watt harnessed steam to meet man's needs for useful power.
Modern hydroelectric power-stations use water power to turn the machines which generate electricity. The water power may be obtained from small dams in rivers or from enormous sources of water power like those to be found in the USSR. However, most of our electricity, that is about 86 per cent, still comes from steam power-stations.
In some other countries, such as Norway, Sweden, and Switzerland, more electric energy is produced from water power than from steam. They have been developing large hydroelectric power-stations for the past forty years, or so, because they lack a sufficient fuel supply. The tendency, nowadays, even for countries that have large coal resources are to utilize their water power in order to conserve their resources of coal. As a matter of fact, almost one half of the total electric supply of the world comes from water power.
The locality of a hydroelectric power plant depends on natural conditions. The hydroelectric power plant may be located either at the dam or at a considerable distance below. That depends on the desirability of using the head supply at the dam itself or the desirability of getting a greater head. In the latter case, water is conducted through pipes or open channels to a point farther downstream where the natural conditions make a greater head possible.