Electricity 233 



FIG. 132. Pencils ready for making an arc light. 



SECTION 38. TJie electric arc. 

 How can electricity set a house on fire ? 



This is one of the most important sections in the book. 



Do you know that you can make an arc light with 

 two ordinary pencils? The next experiment, which 

 should be done by the class with the help of the teacher, 

 shows how to do it. 



Experiment 71. Sharpen two pencils. About halfway 

 between the point and the other end of each pencil cut a 

 notch all the way around and down to the " lead," or burn 

 a notch down by means of the glowing resistance wire. 

 What you call the " lead " of the pencil is really graphite. 

 a form of carbon. The leads of your two pencils are almost 

 exactly like the carbons used in arc lights, except, of course, 

 that they are much smaller. Turn off the electricity both 

 at the snap switch and at the knife switch. Fasten the 

 bare end of a 2-foot piece of fine insulated wire (about No. 24) 

 around the center of the lead in each pencil so that you get 

 a good contact, as shown in Figure 132. Fasten the other 

 bare end of each wire to either side of the open knife switch 

 so that when this switch is open the electricity will have to 

 pass down one wire to the lead of one pencil, from that to 



