THE PULLEY 



467 



It is sometimes exceedingly convenient to change the 

 direction of a force even if no other advantage is gained. 

 To do this, a rope may be passed over a wheel, and thus 

 one may by pulling down lift up the weight. Such an ar- 

 rangement is called a fixed pulley. (Figure 141.) The cord 



COMBINATION OF PULLEYS USED TO LIFT HEAVY BURDEN 



Because of the mechanical advantage of the pulleys, relatively small power 

 is needed to lift this electromagnet, with tons of scrap iron clinging to it. 



in passing around the wheel simply has its direction changed, 

 but there is no gain for the user of the machine either in 

 power or in distance. 



If now the pulley is arranged as in Figure 142, it is no 

 longer a fixed pulley but is movable. It is evident in this 

 case that the weight is supported not by a single cord as in 

 the fixed pulley but by two cords, the part of the cord at- 



