19 



without settiu^ii" (Ire // coiinecicd irilh haij-forh trade, wlu'le the one with points will 

 prevent the flash entirely. 



Metallic Roofs. 



It is sometimes said that a niefallic roof is perfect protection from lightning. 

 If such a roof is put on one of these huildings, it is struck just the same as if 

 it were an ordinary roof. (Fig. 26.) Note flash at stove (which is constructed with 

 air-gaps to cause sparks). It is dangerous jfet) stand by the stove during a thunder- 

 storm. 



If, however, a corner of this roof is connected to the ground by a wire, the 

 current follows the wire. (Fig. 27.) No sparks at the stove now. 



But a flash can be prevented from striking that metallic roof, as in the other 

 cases, in two ways : First, by using lightning rod points ; secondly by putting a sharp 

 edged or notched ridge-board on the top. (Fig. 28.) We might here refer to the 



Fig. 26. — Metallic roof struck by ligut- Pie. 27.— Metallic roof with one corner 



ning. Note flash down front of stove. The grounded. Plash still occurs, but goes down 



stove is constructed with air-gaps in jump- ground wire — no sparks at stove now. 

 ing which current makes sparks. Dan- 

 gerous to stand by stove during thunaer- 

 storm, if house is not rodded. 



practice of using a round metal form on the ridge of a metal roof. Nothing could be 

 more dangerous than this so far as lightning is' concerned unless points of some kind 

 are provided. The round form prevents the electricity from leaking off, and conse- 

 quently tends to store up a charge and cause a stroke. If instead of the round a 

 sharp-edged form or one with teeth were used, it would be almost impossible for 

 a metallic-roofed building to be struck by lightning, if two, or better, four, corners 

 were coimected with the earth by ground wires. 



Wire Fences. 



Here is a wire fence, which is attached to the earth-plate. As the machine is 

 turned a flash of lightning jumps from the cloud to the wire and from the wire to 

 the earth. (Fig. 29.) 



