 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.
 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.
 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.
Detecting a current without a meter 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.
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.  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 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
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. 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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