Resistors

A resistor is one of the most common elements of a circuit. Engineers and scientists use resistors to reduce the value of current in the circuit, to produce IR voltage drop, and in this way to change the value of the voltage.

Current passes through a resistor and its temperature rises high. The higher the value of current the higher is the temperature of a resistor. Each resistor has a maximum temperature. It means that it can work up to these limits without a trouble. If the temperature rises higher the resistor gets open and opens the circuit.

The readings on its scale show the value of resistance in watt. The watt is the rate at which engineers obtain electrical energy when a current of one ampere passes at a potential difference of one volt.

A resistor can have constant value - this is a fixed resistor. The other resistor has different value in different cases. It is a rheostat. Engineers use it to change the resistance of circuits and in this way to vary the value of current.

Electrical cell

Engineers use an electric cell to produce and supply electric energy. It consists of an electrolyte and two electrodes. We use electrodes as terminals. They connect the cell directly to the circuit - current passes through the terminals and the bulb lights.

Engineers connect the cells in series, parallel or in series-parallel. They want to increase the current capacity, they connect the cells in parallel. They connect the cells in series and increase the voltage output. A battery has a large current capacity and a large voltage output, this means that engineers connect its cells series-parallel.

When an engineer connects the cells in series, he connects the positive terminal of one cell to the negative terminal of the second cell, the positive terminal of the second cell - to the negative terminal of the third…and so on. Engineers connect together the cells' negative terminals and positive ones if they want to have the cells in parallel. In case a cell has a trouble it stops or operates badly. Engineer substitutes this cell by another one.

Measurements

Metric system is a decimal system of physical units. It got its name after its unit of length, the meter. The majority of countries adopts the metric system as the common system of weights and measures. The scientists all over the world use this system in their scientific work.

Weights and Measures

We measure length, capacity and weight and use standard units in these cases. The principal early standards of length were the palm or hand breadth, the foot and the cubit, which is the length from the elbow to the tip of the middle finger. Such standards were not accurate and definite. Only in modern time people adopted unchanging standards of measurement.

In the English-speaking world, the everyday units of linear measurement were traditionally the inch, foot, yard and mile. In Great Britain people defined these units of length in terms of the imperial standard yard, which was the distance between two lines on a bronze bar made in 1845.

In Britain scientists now also derive units of weight (ounces, pounds, and tons) from the metric standard — kilogram. This is a solid cylinder of platinum-iridium alloy maintained at constant temperature at Sevres, near Paris.

National standards laboratories in many countries maintain copies of this standard as exact as possible.

International System of Units is a system of measurement units based on the MKS (meter-kilogram-second) system. This international system is commonly referred to as SI.

At the Eleventh General Conference on Weights and Measures that took place in Paris in 1960 scientists defined standards for six base units and two supplementary units.

Length

The meter had its origin in the metric system. By in­ternational agreement the scientists defined the standard meter as the distance between two fine lines on a bar of platinum-iridium alloy. The 1960 conference redefined the meter as 1,650,763.73 wavelengths of the reddish-orange light that the isotope krypton-86 emitted. The scientists again redefined the meter in 1983 as the length of the path that light in a vacuum traveled during a time interval of 1/299,792,458 of a second.

Mass

When scientists created the metric system, they defined the kilogram as the mass of 1 cubic decimeter of pure water at the temperature of its maximum density or at 4.0 °C.

Time

For centuries, we measured time universally in terms of the rotation of the earth. The second is the basic unit of time. Scientists defined it as 1/86,400 of a mean solar day or one complete rotation of the earth on its axis in relation to the sun. They discovered, however, that the rotation of the earth was not constant enough to serve as the basis of the time standard. As a result, scientists redefined the second in 1967 in terms of the resonant frequency of the caesium atom, that is, the frequency at which this atom absorbs energy.