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Electricity-History.

posted Jan 29, 2015, 11:40 PM by ranmini@charliesresearch.com   [ updated Jul 6, 2018, 12:02 PM by Upali Salpadoru ]
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Fig.1  Benjamin Franklin and the kite.



  Electricity  was a mystery for man. He had observed lightening, and attractions produced by rubbing certain substances . It was Benjamin Franklin (1706-1790) USA. who sent up a kite to the clouds and obtained a deadly spark.
 

 In 1780 Luigi Galvani, an eminent surgeon attached to the University of Bolgna in Italy, started a series of experiments to pass electric current, from stored up charges he obtained from lightening, to the tissues of dead animals. His arrangements are shown in Figures 2 and 3 .

                                                                     Fig. 2  Frogs legs hang from a nerve of the spine

Fig3 An outdoor experiment.

One glass insulated iron wire, connected to the dead frog, goes to the roof. The other goes into the well. Galvani found that the legs jerk even when there is no lightening..

Whenever a current was passed the muscles jerked, but what  stunned him was that at times the muscles twitched without any external electricity. He thought that the muscles are capable of producing electricity and called this ‘animal electricity’. As opposed to natural electricity in lightening and  electrostatistics in artificial devices.


 It was Alessandro Volta who made capital use of Galvani’s  discovery.
Following the experiments of Galvani, Volta wanted to find out whether the electric current, originated in the muscles or the metals. In 1794 he decided to check this by using metals without animal cells.
He soon discovered  that an electric current resulted even without any animal tissue.

Fig.4 Voltaic pile.

This sparked a controversy between the two Italians with the German Humboldt, the chief of Galvani's supporters, and the Frenchman Coulomb, the chief of Volta's. The weight of the evidence leaned more and more heavily toward Volta, 

 In 1800 Volta virtually clinched the victory by constructing a device that would produce a continuous flow of electricity. Volta's device was an "electric battery"- the first in history.



Experiment 1
Aim:    Attempts to generate an Electric current.
Method:
Take a plastic or a glass cup and add some salt solution into it. Then dip  two metal plates which are connected to two copper wires. Connect the free ends of the two wires to an electric meter; preferably a Galvanometer or a Mili ammeter.

Fig. 4   Producing a cell to generate an electric current.

The two electrodes, A and B must be two different metals such as iron and Copper. The solution is called the electrolyte.
The chart given below suggests a few substances you may use.  You may use this to record your results too. If the galvanometer needle moves to the right the current flows to the right in the meter and vice versa.

Electrode

A

Electrode

B

Electrolyte

[solution]

Current reading & direction.

Iron

Copper

 

 

Aluminium

 

 

 

Brass

 

 

 

Zinc

 

 

 

Magnesium

 

 

 

Detecting a current without a meter
Many students will not have a suitable meter to detect the current.They still have ways to do it.

Method 1

 The pressure (Voltage) of the current can be increased by coupling a number of cells. The method to do this is shown here. In the set up given below, the wire is expected to get heated if the current flows. The heat will melt the wax and the match stick should fall down.

 

Fig.5. When the current flows the wax melts. 

 Method 2

 Break the circuit at some point and connect a bulb or an LED. If there is a current they will light up. Although a bulb may be connected without considering the direction of the current LED will light up only when the current flows in a particular direction.


Fig.6. When the current flows in a particular direction the LED lights up.

 Electric Current – Modern view

 A current is usually movement of matter. We talk of water currents and air currents. Similarly an electric current is also a kind of movement.

  •    Is it matter?


 This is debatable. For matter to go through there must be some space. PVC pipes that carry water have a groove inside. The lines and wires that carry current are quite solid.

  •   If so what kind of a movement is an electric current?
The only possibility is that, what ever that moves, must be inside the conductor.   Atoms are the basic building blocks of all matter. Sir Ernest Rutherford has shown the arrangement of particles called “Sub Atomic Particles” or “Fundamental Particles” inside an atom.Electrons are negatively charged particles that go round the nucleus of atoms. Occasionally they can get detached and move away in the free space between the atoms.

Protons carry the opposite charge and they lie in the center held together by a powerful force. They can merely vibrate. A change in the nucleus of an atom is a thing of much concern.
Neutron do not have an electrical charge although they have a mass. These are also embedded in the nucleus.
The model of an atom according to Rutherford is shown  in Figure below.

 

Fig. 7. Magnesium atom has 12 orbiting electrons in three shells.

If the atoms have less than 2 electrons in the outer shell they are good conductors.

                                                                           Visit the Periodic Table 

 

  Sign Convention

 At first scientists knew very little about electricity. They did not know what was flowing   and which way they were flowing. Benjamin Franklin suspected a flow .and assumed that the flow is from positive to negative.     

 We still maintain this sign convention although, the electrons flow from negative to positive.

 Home Experiment   2
Aim: To separate substances into conductors and Insulators.
Method:-
        

A bulb is connected to a battery and the two ens of the wire are tightly wound round two screw drivers.  In this circuit the current will not flow as the two electrodes join only by the particles of air.     As air is a very bad conductor [ very good insulator] the bulb will not light. The blades of the two screw drivers have to be pressed on the materials to be tested. If the bulb lights at normal brightness  the material must be a good conductor. if there is a slight glow it may be passing a small current. If it does not light at all the material must be an insulator.


Fig.8.  A method to select conductors.

 Results

Substance

Good Conductor

Conductor

Insulator

Iron

 

 

 

Brass

 

 

 

 You may also test Aluminium,Wood,Plastic,Glass,Clay.Water,Oil and 

solutions such as salt solution, dilute acids and alkalis.

 Voltaic Cell

The type of cells we used to obtain current in the experiments are called Simple cells or Voltaic cells. Now it is time to find out how they generate a current. Zinc is a metal that react with dilute sulphuric acid.
        Zinc + sulphuric acid= zinc sulphate + Hydrogen
As the  zinc dissolves electrons migrate along the wire. The sulphric acid solution has Hydrogen ions with a positive charge and sulphate ions. The positive hydrogen ions take electrons at the copper plate and become hydrogen atoms.
 Hydrogen bubbles form at the copper plate. The reaction will retard due to these bubbles. This defect is called polarization. If you wish to continue, the bubbles must be removed with a brush or by some other method.

Electric Lighting


In 1802, Humphrey Davy invented electric battery with which he obtained a brilliant light getting a continuous spark using carbon rods.In 1840, Warren de la Rue (Br)  used a platinum coil in a vacuum filled bulb. This did not become viable as platinum was too expensive.


In 1850 Joseph Wilson Swan(Br) using the same technique used partially burnt paper instead of a platinum filament. Although this functioned well the carbon got burnt due to the poor quality of the vacuum. After two decades he got better evacuating pumps and obtained an efficient source of light using a treated cotton thread as the filament.

In 1874, Woodward a medical student and his partner, Mathew Evans, obtained a Canadian patent. It was a glass tube filled with nitrogen with a piece of carbon. Although it functioned well they could not commercialize it. Then they sold the patent to Thomas Alva Edison after 5 years.

 In 1879, Thomas Alva Edison obtained U.S. patent for a vacuum filled bulb using a carbon filament which he later improved to a bamboo filament which continued to glow for over 1000 hours.

Fig. Thomas Alva Edison. 

1847 - 1931 . US

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