Conducting materials

Solids are classified into three-conductors, semiconductors and insulators. In conducting materials, for example metals, valence electrons are free to move about the lattice and are known as free electrons. Whenever these free electrons suffer scattering, resistivity rises. There are two causes for scattering. They are scattering due to impurities present in the sample and scattering due to lattice ion vibrations. Conducting materials possess very feeble resistivity and high conductivity. Power loss and voltage drop are very less in these materials. Temperature co-efficient of resistance is very low for conducting materials. They are highly ductile and malleable. Soldering and welding of conducting materials are easy. Conducting materials are normally very strong and can withstand stress and strain. Conducting materials have high resistance to corrosion when exposed to atmosphere. Some examples of conducting materials are Aluminium, Brass, Bronze, Copper, Gold, Mercury, Silver and Zinc. Commonly used conducting materials are Copper, Aluminium, Brass and Bronze. High resistance conducting materials like tungsten, manganin, constantan, nichrome, carbon, platinum and mercury are used in electric lamp filaments, electron tube heaters, radars, electronic valves, rheostats and motor starters.

Cross Drainage

Cross drainage work involves the construction of a drainage system at the crossing of a canal which helps to dispose drainage water without interrupting the continuous canal supplies. The cross drainage work can be reduced but never avoidable. reduction in cross drainage work is possible by the alignment of artificial canals along the ridge line called water-shed. Cross drainage works are not essential once the canal reaches the water shed line. Aqueduct and symphon aqueduct is a type of cross drainage work, where the canal is taken over the natural drain in such a way that the drainage water runs below the canal either freely or under syphoning pressure. In super-passage and symphon method, the drain is taken over the canal in such a way that canal water runs below the drain either freely or syphoning pressure. In the level crossing method, the canal water and drain water are allowed to intermingle with each other. Choice of cross drainage work depends on many factors such as  suitable canal alignment, nature of available foundation, position of water table, availability of dewatering equipments, head loss in canal, suitability of soil for embankment and of course the availability of funds.  

Study of cell ultrastructure

A cell is a basic unit of any organism. Its volume ranges from a few attolitres (bacteria) to milliliters (nerve cells of squid). Structure and metabolic activities changes from cell to cell. Cells usually assume flexible structures with a delimiting dynamic membrane in an undulating state. Diffentiation is a process where the development of a stem cell into cells with spaecialized function takes place. For example, in the case of a fetus, single cell is resulted from the fusion of one spermatozoon and one ovum. This fusion leads to the formation of a vast array of different tissues. For the study of structure and functioning of cells, many methods are used. They are optical microscopy, electron microscopy, histochemistry, cytochemistry, autoradiography and ultracentrifugation. Optical microscopes use light and can be used to identify cells. But fine structure of cells and very tiny cells are not coming to the frame optical microscopes. For this, electron microscopes are used. There are many types of electron microscopes. There are many varieties in electron microscopes. Some of them are scanning electron microscopes, transmission electron microscopes and scanning probe microscopes. Histochemistry deals with whole tissues and cytochemistry with individual cells. Autoradiography is a method used for identifying and locating radioactive compounds within cells. Ultracentrifugation is a method of fractionation of the cells which is required for the study of biochemical activity of subcellular organelles.