LEVER 



3392 



LEVER 



has a bubble of air in a glass tube containing 

 alcohol or some other spirit. The tube is 

 slightly curved upward toward its center, so 

 when the instrument is in a horizontal posi- 

 tion the bubble, rising to the highest point, 

 will be exactly at the center. In a carpen- 

 ter's or mason's level the spirit tube is con- 

 tained in a wooden bar. A second tube is 

 often placed across the end of the instrument, 

 for testing vertical surfaces. In using such a 

 level it is well to reverse it after the first test, 

 because of possible inaccuracy in its construc- 

 tion. If the bubble is at the center line in 

 both tests, the instrument is reliable. Survey- 

 or's leveling instruments usually contain spirit 

 levels. 



An older form of carpenter's level was a 

 plumb-line fastened to a T-shaped frame. 

 When the line hung parallel with the stem of 

 the T, the crossbar was known to be horizon- 

 tal. 



LEVER, le'ver, or lev' er, in physics, one of 

 the simple machines by which work is accom- 

 plished. It consists of a rigid bar which can 

 be moved about a fixed point, and is illustrated 

 by the familiar seesaw of the children's play- 

 ground. A plank is balanced across a saw- 

 horse, and a child sits at each end of the 

 board; as the board is set in motion, work is 

 done, for the children move up and down. In 

 this case the support upon which the board 

 rests represents the fulcrum of a lever; the 

 parts of the board between the children and 

 the support are the arms; the weight at the 

 end which is in the air represents the force 

 used to move the board and is called the 

 power, and that at the other end represents 

 the object to be lifted, or the weight. The 

 part between the fulcrum and the point where 

 the power is applied is called the power arm; 

 that between the fulcrum and the weight is 

 the weight arm. 



If you multiply the weight by the distance 

 it is moved, you have the amount of work 

 done by the lever (see FOOT-POUND). If you 

 multiply the power exerted by the distance it 

 travels in doing the work, you have the same 

 result. It is plain, therefore, that if the power 

 moves a shorter distance than the weight, it 

 must be greater than the weight, but if it trav- 

 els a longer distance, it may be less than the 

 weight. This principle is what determines the 

 relative efficiency of the three classes into 

 which levers are divided, according to the posi- 

 tions of power, weight and fulcrum. These are 

 explained, as follows: 



First Class. Levers of the first class have 

 the fulcrum between the weight and the power, 

 as in the seesaw, crowbar, balance scale, pump 

 handle and catapult. Scissors and pincers are 

 double levers of the first class. If the two arms 



A LEVER OF THE FIRST CLASS 

 P is the power ; W, the weight ; F, the ful- 

 crum A pump handle and a crowbar may repre- 

 sent the figures at the left, and at the right is 

 shown the mechanical value of the crowbar. If 

 the distance from W to F is half that from F to 

 where P is applied, P (or the power in pounds) 

 need be only half that of W in order to move W. 



of the lever are equal, the power must equal 

 the weight ; if the power arm is longer than the 

 weight arm, as in the crowbar, so that the 

 power travels farther than the weight, the 

 power can be less than the weight. 



Second Class. In this division the weight is 

 between the power and the fulcrum, therefore 

 the power always travels a greater distance 

 (as in the wheelbarrow shown in the. sketch) 

 and is less than the weight. An oar is another 



A LEVER OF THE SECOND CLASS 

 In lifting the weight (W) the power (P) is 

 always less than W, because it is farther from F 

 and moves a greater distance. 



example ; a nutcracker is a double lever of this 

 class. 



Third Class. Levers of this class have the 

 power between the weight and the fulcrum. 

 As the positions of weight and power are ex- 

 actly opposite to their positions in the second 



