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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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Periodic Classification of Elements :

 

"The arrangement of elements in their blocks,periodic and groups is called classification of elements.

Dobereiner's Classification :
 

In 1829, W.Dobereiner, a German Chemist,made use of Idea of relationship between atomic masses and properties of elements for the classification of elements.He arranged similar elements in to groups of three elements call "triads".

Law of Triads :

"The atomic mass of the middle element was found to be approximately the average of the other two elements of triads". This is know as "Law of Triads"

Li = 7
Na + 23+39/2 =23
K = 39


This classification could not be extended to all elements for their classification.

New Land's Classification :

In 1864.New Land  an English chemist attempted the classification of elements in order of their increasing atomic masses.He observed the properties of elements & found that properties of elements.

Law of Octave :

"The properties of every 8th element from the given one were similar to the first elements"

He called this periodicity as the law of octave and arrangement the elements accordingly.

Advantages and Disadvantages of Law of Octave :

1.This law provided a basis for the classification of elements in to groups of elements having similar properties.
2.This law provided a wider scope to arrange all known elements in to a tabular form.
3.The periodic arrangement of elements did not include noble gases because they were not discovered then ; and the heavier elements could also not be accommodated.

Lother Meyer's Classification:


In 1869,Lother Meyer,a German chemist arranged the elements in order of this increasing atomic masses and found that physical properties of elements were the periodic function of their atomic masses and found that physical properties of elements were the periodic functions of their atomic masses.

He plotted a graph between atomic values of the elements against their atomic masses and found that similar elements occupied similar positions in the graph.

Mendeleev's Classification:


In 1869, Mendeleev, a russian chemist discovered a rule for the classification of elements.He found that if elements are arranged in order of their increasing atomic masses,their properties vary in a regular manner, from one member to another in a series,In other words elements show periodicity in their properties.

Mendeleev's Law:

"Physical and chemical properties of elements are periodic functions of their atomic masses"

This is called Mendeleev's Law.Mendeleev's Periodic table was very precise and formed the basis of modern classification of elements.

Modern Classification of elements:

In 1913,Moseley a British physicist found that atomic number is the fundamental property of an atom .With this discovery it was found that when elements are arranged in ascending order of their atomic numbers, the elements possessing similar properties are repeated in a regular manner.

Periodic Law:

"Physical and chemical properties of elements are the periodic functions of their atomic number.This is periodic Law.

 









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