256 



ELECTRICITY 



devices. In industry, electric heat is used in bakeries, 

 in laundries, and in the manufacture of many common 

 substances such as carborundum and steel. The ovens 

 in which the large block of special glass for the 200- 

 inch reflecting telescope was made were heated by 

 electricity. It was necessary to allow the cooling of 

 this glass block to go on very slowly and evenly. Elec- 

 tric heat was the only heat in which the precise con- 

 trol of the temperature could be obtained. 



Devices which transform electrical energy into heat 

 energy usually have heating elements made from 

 some material which offers so much resistance to the 

 flow of the electric current that it will become red 



FIG. 406. DEVICES WHICH CONVERT ELECTRICAL 

 ENERGY INTO OTHER FORMS OF ENERGY 



hot. Many patented alloys for these heating elements 

 are in use. They vary in their chemical composition, 

 but usually contain nickel, chromium, and iron. Ex- 

 amine the heating element of an electric heater or 

 toaster and note the color and general characteristics 

 of the material from which it is made. 



How do electric lights work? In flashlights, auto- 

 mobiles, and homes and for street lighting, electric 

 lamps of every description are used. 



The earliest experiments in changing electrical 

 energy into light energy were made by Sir Humphry 

 Davy, a great English scientist, tie discovered that 



FIG. 407. SIMPLE ARC LAMP 



when current from his large battery was brought 

 through two pieces of carbon, which were afterwards 

 slightly separated, a brilliant light was produced. 

 Later this discovery was developed into the modern 

 arc lamp. When the carbons in Figure 407 are touched 

 and then slightly separated, 

 current continues to flow 

 across the air gap between 

 them, giving a very bright 

 light and a great amount 

 of heat. This type of light 

 was very common a few 

 years ago for street light- 

 ing and now is used widely 

 in one type of health or 

 "sun-ray" lamp. 



After the development 

 of the electric generator, 

 Thomas A. Edison turned' 

 his attention to the problem 

 of producing a device which could change electricity 

 into light. After long and careful experimenting, often 

 with most discouraging results, he produced in 1879 

 the first electric lamp. In the development of this 

 lamp many perplexing problems had to be solved, 

 such as finding a substance which would last for some 

 time as a filament, getting the electricity into the 

 globe, and removing air from it. 



Carbonized cotton was finally found to be a suit- 

 able material for the filament, and after the air from 

 the bulb was removed to prevent the filament from 

 burning, a lamp was successfully operated for a period 

 of forty hours. 



In the fifty years which have passed since this his- 

 toric experiment, great progress has been made in 

 electric lighting. Tungsten, a metal with a very high 

 melting point, has replaced carbon as the filament. 

 Gases which will not combine with the hot filament 

 are put into the globe after most of the air has been 

 removed. Globes are now frosted on the inside to 

 prevent glare. Truly great advances have been made, 

 but what a debt we owe to the great mind that first 

 made these developments possible. 



FIG. 408. HENRY'S DIAGRAM OF A MOTOR 



