no. 2 history of electric light schroeder 7 



ohm's law 



Georg Simon Ohm was born in Bavaria, the oldest son of a poor 

 blacksmith. With the aid of friends he went to college and became 

 a teacher. It had been shown that the rate of transfer of heat from 

 one end to the other of a metal bar is proportional to the difference of 

 temperature between the ends. About 1825, Ohm, by analogy and 

 experiment, found that the current in a conductor is proportional 

 to the difference of electric pressure (voltage) between its ends. 

 He further showed that with a given difference of voltage, the current 

 in different conductors is inversely proportional to the resistance of 

 the conductor. Ohm therefore propounded the law that the current 

 flowing in a circuit is equal to the voltage on that circuit divided by 

 the resistance of the circuit. In honor of this discovery, the unit of 

 electrical resistance is called the Ohm. This law is usually ex- 

 pressed as : 



"C" meaning current (in amperes), "E" meaning electromotive 

 force or voltage (in volts) and "R" meaning resistance (in ohms). 



This is one of the fundamental laws of electricity and if thoroughly 

 understood, will solve many electrical problems. Thus, if any two of 

 the above units are known, the third can be determined. Examples : 

 An incandescent lamp on a 120-volt circuit consumes 0.4 ampere, 

 hence its resistance under such conditions is 300 ohms. Several 

 trolley cars at the end of a line take 100 amperes to run them and the 

 resistance of the overhead wire from the power house to the trolley 

 cars is half an ohm ; the drop in voltage on the line between the power 

 house and trolley cars is therefore 50 volts, so that if the voltage at 

 the power house were 600, it would be 550 volts at the end of the line. 



Critics derided Ohm's law so that he was forced out of his position 

 as teacher in the High School in Cologne. Finally after ten years 

 Ohm began to find supporters and in 1841 his law was publicly 

 recognized by the Royal Society of London which presented him with 

 the Copley medal. 



INVENTION OF THE DYNAMO 



Michael Faraday was an English scientist. Born of parents in 

 poor circumstances, he became a bookbinder and studied books on 

 electricity and chemistry. He finally obtained a position as laboratory 

 assistant to Sir Humphry Davy helping him with his lectures and 



