220 ELEMENTS OF STATIC ELECTRICITY. 



Twisted wire cables have the advantage of compara- 

 tive freedom from joints ; but their interstices afford a 

 lodgment for smoke, dirt, and water; especially if 

 small wires are used: which are also less capable of 

 resisting oxidation than large wires. 



Tape has the special advantages of requiring but few 

 joints; of their being easily made, where necessary; 

 and of being flat and flexible, so that it can be adapted 

 to the outlines of a building, or countersunk in it and 

 painted over, so as not to be conspicuous. 



Conductors should not be bent abruptly round sharp 

 corners : and in no case should the length of conductor 

 between the two points of a bend be more than one- 

 half greater than the straight line joining them. When> 

 practicable, the conductor may pass straight through a 

 projection; the hole being made large. enough to allow 

 it to pass freely, without compression. 

 x The reasons for these precautions are found in the 

 liability to discharge from a sharp angle, or across a 

 short space in a bend. 



JOINTS. The most fruitful source of danger in con- 

 ductors is from bad joints. Screwed, scarfed, or riveted 

 joints, however well made, are certain to rust and cor- 

 rode in time ; introducing nodes of resistance, at which 

 the electric charge is liable either to fuse the conductor, 

 or to leave it and enter the building. 



No joint is electrically perfect that is not metallically 

 continuous, and as absolutely free from resistance as 

 any other part of the conductor : and careful soldering, 

 in addition to the screwing, scarfing, or riveting, is the 

 only certain means of securing this, which has borne 

 the test of experience. 



EARTH CONNECTION. A good earth connection, for 



