15. Steam turbine

Steam is the principal factor in producing usable energy because of the power created by its expansion. The discovery of the power in steam produced great changes in industry. '£ Steam power is used mainly in the generation of electric­ity. At present only about 25 per cent of the power used in the Soviet Union is obtained from moving or falling water. Besides electrical power plants are needed in many places where water power is not available. A great number of electric power stations throughout the country is run by the me­chanical power of steam turbines.

Here is an example of an invention that had to wait many centuries before men discovered its practical application. We mean the steam turbine. More than two thousand years ago a man named Hero who lived in Alexandria, Egypt, made the first steam turbine. It was a steam engine that produced rotary motion. It used neither a piston nor a cylin­der as is the case in steam engines. The steam from a boiler was carried into a ball which had a pair of bent tubes. The steam forced the ball to rotate.

.' For almost two thousand years men did little with this idea. They did not know how to make tjie engine perform useful work. Then a few hundred years ago, men began to experiment with this device and came to a simple steam turbine. The ro­tor mounted on a shaft has many small blades around its out­side edge. Nozzles direct jets of steam against these blades causing the rotor to rotate at high speed. This rotary mo­tion can be taken off the rotor shaft by gears to drive other machinery or an electric generator.

A modern turbine usually has not just one but a whole series of bladed disks which together make up the rotor. They are all mounted on a single shaft so that all rotate together. Between each pair of disks there is a stationary ring in which a series of blades is set. The blades curve in the opposite direc­tion from the blades in the rotor. After the steam has passed through the first disk and given the rotor a powerful rotary push, it reverses direction. It then goes through the curved blades on the stationary ring which reverses its direction again, so that it can push against the blades in the second disk. In this way, the steam goes through the turbine, pushing against the rotor blades and changing direction again in the stationary blades and so on. Most of the useful energy in the steam is utilized by the above process.

16. Units of measurement

' In measuring the rate at which electrons are moving through a conductor, an electrician could say that the electric current is flowing at the rate of one coulomb per second. How­ever, electricians have a unit which measures it directly and, therefore, instead of using the above-mentioned expression, an electrician would simply say: the current is one ampere. The ampere is the electrical unit which measures directly the quantity of electricity flowing in the conductor. The kiloampere, the largest unit of current, is equal to one thou­sand amperes. Where the ampere is too large a unit to be used, we may employ the milliampere or the microampere, the prefix "milli" meaning a thousandth and "micro" standing for a millionth.

Keep in mind exactly what a volt is because the term is constantly used in all branches of electrical work. It is the practical unit used to measure the pressure that causes the electric current to flow through the circuit. However, it is necessary to have both larger and smaller units. Thus, we have a megavolt (million volts), a millivolt (a thousandth of p volt), and a microvolt, that is a millionth of a volt. 3 In electrical circuits, we are also interested in the magni­tude of the resistance in each conductor. Resistance plays a very important part in the operation of every electrical cir­cuit. For that reason, it became necessary that some special practical unit be developed. It would indicate definitely how much resistance were present in any given conductor or cir­cuit. That unit is called the ohm, a megohm equalling one million ohms and a micro-ohm betn^-orre^millionth of an ohm. • The ohm was named after an experimentor who investigat­ed the resistance taking place in electrical circuits. His name was George Simon Ohm. He carried on numerous experiments which demonstrated that there is a very close relationship between voltage, current, and resistance in any given circuit. He showed that the amount of current which flowed in a cir­cuit depended both upon the amount of resistance in the circuit and the amount of voltage which caused the current to flow.

Having considered the measurement of electrical quanti­ties, we shall define now two units of heai. These are the calorie and the British thermal unit. The first is a metric unit and may be defined as the average amount of heat required to raise the temperature of one gram of water one degree Centigrade. In the same way/ the British thermal unit, or Btu, is the average amount of heat necessary to raise the tem­perature of one pound of water one degree Fahrenheit. Since the calorie is a rather small quantity of heat, a large unit called the kilogram calorie, or large calorie, is often used. It is not difficult to understand that the kilogram calorie is 1,000 times as large as the calorie which was defined above.