50 THE PRODUCTION AND USE OF LIGHT 



automobiles holds 40 cu. ft. of gas which sells at about 

 cts. per cu. ft. 



63. Danger of Acetylene Explosions. The gas is highly 

 explosive. Its explosive limit with air is very wide. Air 

 containing over 2^ per cent, of acetylene by volume and under 

 55 per cent, will explode. With gasoline gas the limit is 

 narrow, from 2 per cent, to 5 per cent. Since acetylene 

 generators are usually placed in the cellar or basement of the 

 house, the danger of acetylene accumulating in the air of the 

 cellar through leakage from the generator is rather great, 

 especially with poorly constructed machines and careless 

 handling. If such a mixture should be ignited by striking a 

 match in the cellar or by other means, the result would be 

 disastrous. Generators that stand outside the house and in 

 the open air are now on the market. They are protected 

 against freezing in the winter. 



XI. ELECTRIC LIGHTING 



64. Heating Effects of the Electric Current. Whenever an 

 electric current passes along a wire, the wire becomes more or 

 less heated. It may not become very hot, but it would if it 

 were not sufficiently large or made of the right kind of material 

 to carry that amount of current. A copper wire will carry, 

 without becoming perceptibly heated, a current of electricity 

 which would heat to a high temperature a wire of iron or 

 German silver of the same size. 



Exercise 23. Heating a Wire by Means of an Electric Current 



Scrape the insulation off the ends of some No. 32 German silver 

 wire. Loosen the burr, or nut, on one of the binding posts of a fresh 

 dry cell. Slip one end of the wire under the burr and turn the burr 

 down tight upon it. Be certain that none of the insulation comes 

 between the wire and the binding post and burr. Now loosen the burr 

 on the other binding post. Grasp the loose end of the wire and draw 

 the bared portion under the burr of the second binding post and turn 

 the burr down tight. If the cell is fresh, the wire will become hot; 

 the insulation will begin to smoke and char, and probably will actually 

 burn. The shorter the wire, the hotter it will become (Fig. 35). 



