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. How-

ever, 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 electric current to flow through the circuit. However, it is necessary to have both larger and smaller units. Thus, we have megavolt (million volts), millivolt (a thousandth of a volt), and micro­volt, that is a millionth of a volt.

In electrical circuits, we are also interested in the magni­tude of the resistance in each conductor. I 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 which 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 arid a micro-ohm being one millionth of an ohm.

The ohm was named after an experirnentor who experi­mented with the phenomena of 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 heat. These are the calorie and the British thermal units. 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 larger" unit called the kilogram calorie, or large calorie, is often used. It is not difficult to understand that the kilogram calorie is 1000 times as large as the calorie which was defined above.

ELECTRON THEORY

The foundations of the modern theory of electricity were laid in the study of the electric discharge through gases, and in particular the so-called cathode rays. The nature of these cathode rays was first described by Crookes (1879) when he considered them as negatively electrified particles which were emitted from a metal under the influence of a strong electric field.

Further experiments made on these particles confirmed that they carried a negative charge and the name "electron" was given to them.

It is the movement of these electrons, whether in a conduc­tor or gas which gives rise to the phenomenon known as the electric current. The number of the electrons comprising the unit of current has been computed. At present, we know one microampere to be equal to the passage of 6 milliard electrons per second.

To keep a 100-watt lamp burning requires a flow of six milliard milliard electrons—not in a day, nor an hour, but every second. Six milliard milliard means the figure six with eighteen zeroes after it.

The reader should remember that an electron, being nega­tively charged, will move towards that end of the circuit or that part which is termed "positive". The old convention of the electric current flowing from the positive pole or end of the circuit to the negative was accepted long before the existence of the electron theory. It is in direct opposition to the real —direction of flow of electrons. The convention is, however, too firmly established and the current is still assumed to flow from positive to negative.

STEAM POWER

Steam is the principal factor in producing usable power 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. There are, however, many other examples of steam-operated machines. There are two main types of steam machin­ery: the reciprocating engine and the turbine. In the former, steam pressure pushes against a piston connected with a crank that converts the forward and backward movement into rotary motion. In the second or turbine type, the operation is sim­ilar to that of water turbine. Jets of steam under high pressure hit the blades on the turbine wheel causing rota­tion.

The reciprocating engine develops high power at low speed while the turbine develops high power at high speed. The recip­rocating engine is often used to pump great quantities of water. The turbines are generally used in steam-electric gener­ating plants where high speed as well as great power is neces­sary. The reciprocating engine is from ten to thirty per cent efficient. Turbines usually are much more efficient because they allow a more complete expansion of steam than do recip­rocating engines.

ELECTRIC METER

How would you measure electricity? You can weigh coal. You can count apples. You can measure milk. But it is quite different with electricity because you cannot even see it. Electricity does not weigh anything. How do you measure electricity?

Well, that was not an easy question to answer, even for the scientists who tried all kinds of ways to find a suitable arrangement. But the problem was finally solved, and if you want to see how, look at your electric meter.

It does more than just measure current. It multiplies current times voltage, which is not an easy thing to do when you remember that the voltage is changing from 127 volts positive to 127 volts negative and back again 50 times a second- -. The current is also changing all the time with the demands of your electric appliances-./The multiplication of current times voltage gives watts, which is a measure of electric power.

Having the watts all figured out at any instant, the meter multiplies those by the length of time they are being used. This gives an answer in watt-hours, which is a measure of electric power. Then as if that were not enough, the meter divides by 1000 and shows the final result of the calculation on a set of dials at any and every instant in terms of kilowatt-hours—the units in which you get electric power.

Knowing what it has to do, one might expect a meter to be as big as a piano. But as everybody knows, it is not. Instead of it, itis a little box, starting its arithmetic lesson when you turn on a light, stopping when you turn it off.

BOILING

If we heat some water in an open glass container, we can see that evaporation goes on from the top surface. This evapor­ation is indicated by the clouds forming where the vapour mixes with the colder air and condenses. We find that the temperature of water gradually rises until the thermometer registers 100°C. A little before this point is reached, bubbles appear on the sides of the container. They consist partly of gases driven from liquid and partly of water-vapour, for eva­poration is directed into the bubbles. Water is said to boil when vapour is formed both at the bottom of the container and at the top of it. The motion of the boiling water is caused by the bubbles of vapour rising through the water. The temperature of the boiling water is constant. This temperature is known as the boiling point of the liquid.

The boiling point of a liquid is the temperature at which it boils under some given pressure. When this point has been reached, further heating does not increase the temperature of the liquid, but only changes it into steam.

When water boils in a container we say that we see steam coming out of it. In fact what we see is not steam at all but fine water particles. Steam itself is invisible. It is the con­densed steam in the form of fine particles of water that we see.

As liquids always increase in volume when passing into the state of vapour, an increase in pressure always produces an increase in the boiling point.

Just as solids may under certain conditions be cooled below their melting points without freezing, so liquids may be heated above their boiling points without boiling.

LAWS OF BOILING

The principal laws of boiling are as follows: 1. When a liquid is heated, it begins to boil at a definite temperature, known as the boiling point, and on further heating the temperature remains constant at this value until the whole of the liquid is converted into vapour.

2. This temperature is constant for a given liquid if the pressure is constant.

3. The boiling point of a liquid increases if the pressure upon it is increased.

4. A definite quantity of heat is required to convert the unit mass of the liquid into vapour at the same temperature. This is known as the latent heat of evaporation.