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ELECTRICAL MEASURING INSTRUMENTS For the detection or measurement of electric current, potential difference, and resistance certain instruments have been devised viz. The galvanometer for the detection of small currents measurements of small currents of the order of micro amperes or mili amperes the voltmeter or potentiometer for the measurement of potential difference (and voltage) between two points of a circuit are the EMF of a source the ammeter for the measurement of large currents the wheatstone bridge the meter Bridge the post office box and the ohmmeter for the measurement of resistance. THE MOVING COIL GALVANOMETER The moving coil galvanometer is a basic electrical instrument it is used for the detection (and Measurement) of small currents. Its underlying principle is the fact that when a current flows in a rectangular coil placed in a magnetic field it experiences a magnetic torque. If it is free to rotate under a controlling torque, it rotates through an angle proportional t...
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REFINING OF PETROLEUM 

Petroleum is refined by distillation.Crude oil is carried by pipes to a fractionating column and heated to above 400 C.A number of products are obtained at different temperatures and condensed.

More than 500 compounds have been formed in petroleum distillate boiling below 200 C.Gasoline or petrol is a mixture of hecane (C6H14) and heptane (C7H16) and Heptane (C7H16) Since the demand of gasoline is increasing due to to its increased use a fuel for automobiles, airplanes etc.The cracking is employed to obtain increased quantities of petrol from crude oil.

REFORMING OF PETROLEUM :-

Knocking is a sharp metallic sound produced in the internal combustion engine.Its a cause is gasoline of low octane number.The octane number of gasoline is improved by reforming,a process some what similar to cracking,which again brings about the arrangements of straight chain alkanes into branched chain alkanes.If the petrol used is of such quality that its vapour mixed with air can ignite prior to high compression on passing of firing spark,the energy produced by burning of fuel is not utilized in moving the piston.By the use of catalyst, the straight chain hydrocarbon molecules may be reformed to other molecules which give better performance as motor fuels.This is particularly important in the manufacture of aviation gasoline,In the presence of compounds called antiknock compounds, the knocking can be decreased.Value of the fuel is expressed in terms of its octane number.The octane number of gasoline may also be increased by adding tetrathyl lead Pb (C2H5)4 as knock inhibitor which moderates the explosions in the cylinders of the engine.But fuels containing Pb(C2H5),pollute the atmosphere with lead.

CRACKING :-

The fractional distillation of petroleum yields only about 20% petrol.Additional quantities of petrol are obtained by cracking of higher boiling fractions.Cracking involves splitting the larger molecules of the less volatile fractions into smaller molecules of greater volatility by subjecting them to high temperatures and pressure in the presence of catalyst.Besides increasing the yield of petrol from petroleum, cracking also yields large amounts of useful by products such as ethene, propane, butene and benzene.These are used for making drugs,plastics,detergents,synthetic fibers,synthetic rubber,fertilizers weed killers and important chemicals like ethanol,phenol and acctone.     

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