NO. 2 HISTORY OF ELECTRIC LIGHT — SCHROEDER 1 7 



The rate of energy (power) is the energy divided by a unit of time ; 

 thus one horsepower is 33,000 foot-pounds per minute. Joule next 

 investigated the relation between heat and electric current. He made 

 a device consisting of a vessel of water in which there were a ther- 

 mometer and an insulated coil of wire having a considerable resistance. 

 He found that an electric current heated the water, and making many 

 combinations of the amount and length of time of current flowing 

 and of the resistance of the wire, he deduced the law that the energy 

 in an electric circuit is proportional to the square of the amount of 

 current flowing multiplied by the length of time and multiplied by the 

 resistance of the wire. 



The rate of electrical energy (electric power) is therefore propor- 

 tional to the square of current multiplied by the resistance. The 

 electrical unit of power is now called the Watt, named in honor of 

 James Watt, the Englishman, who made great improvements to the 

 steam engine about a century ago. Thus, watts = C'R and substituting 



E 

 the value of R from Ohm's law, C = ^, we get 



Watts = Volts X Amperes 



The watt is a small unit of electric power, as can be seen from the 

 fact that 746 watts are equal to one horsepower. The kilowatt, kilo 

 being the Greek word for thousand, is looo watts. 



This term is an important one in the electrical industry. For 

 example, dynamos are rated in kilowatts, expressed as KW ; the largest 

 one made so far is 50,000 KW which is 66,666 horsepower. Edison's 

 first commercial dynamo had a capacity of 6 KW although the terms 

 watts and kilowatts were not in use at that time. The ordinary sizes 

 of incandescent lamps now used in the home are 25, 40 and 50 watts. 



Starr's incandescent lamp 



J. W. Starr, an American, of Cincinnati, Ohio, assisted financially 

 by Peabody, the philanthropist, went to England where he obtained 

 a patent in 1845 on the lamps he had invented, although the patent was 

 taken out under the name of King, his attorney. One is of passing 

 interest only. It consisted of a strip of platinum, the active length of 

 which could be adjusted to fit the battery strength used, and was 

 covered by a glass globe to protect it from draughts of air. The other, 

 a carbon lamp, was the first real contribution to the art. It consisted 

 of a rod of carbon operating in the vacuum above a column of mercury 

 (Torrecellium vacuum) as in a barometer. A heavy platinum wire 



