90 



THE SCREW. 



The screw is, like the wedge, only a particular form 

 of the inclined plane. If a right-angled triangle, cut 

 out of paper, be wrapped round a cylindrical body, for 

 instance a test-tube (fig. 64, A), the slope of the 

 inclined plane will represent a spiral line, called the 

 thread. A screw is a cylinder with a spiral ridge 

 raised upon it. This ridge may be either angular (fig. 

 64, B, D), or square (fig. 64, C). To use the screw it 

 is necessary to have a hollow cylinder with a groove cut 



FIG. 64 (real size). 



on the inside of it, so that the thread of the screw exactly 

 fits into it. This hollow cylinder is called the nut. If 

 the nut is fixed while the screw is turned, the latter 

 passes through the nut. The converse takes place if the 

 screw is fixed and the nut is moveable. Screws like B 

 and (?, in which each turn of the thread appears higher 

 on the right-hand side than on the left when the screw 

 is held upright, are called right-handed screws. Screws 

 like D, in which the left end is higher, are called left- 

 handed. The former kind is much more often used 

 than the latter. In the screw one of the forces acts 

 in the direction of the axis of the cylinder, the space 



