THE LEVER AND BALANCE 



41 



This rule of universal application for taking moments will have to 

 be used several times later on, and should be well borne in mind. 



The moment of a force about any point is the product obtained by 

 multiplying the force by the perpendicular distance between the point and 

 the line of action of the force. 



17. THE BALANCE AND ITS USE. 



i. The balance. (a) Uncover the balance and identify the different parts 

 by reference to Fig. 32. Raise the beam, AB, of the balance, off the 

 supports by turning the handle C. Notice whether the pointer F swings 



FIG. 32. The Student's Balance. 



equally on both sides of the middle of f he scale G : if it does the balance is 

 ready for use ; but if not let down the beam and turn the small screw at B 

 then try again. Repeat this adjustment until the swings to right and left 

 are equal. 



(6) When using the balance always place the substance to be weighed 

 in the left-hand pan, E, of the balance. Examine the box of weights, and, 

 selecting a weight which you estimate to be about the same as that of an 

 object placed in the pan, E, take hold of it with the forceps and place it in 

 the right-hand pan. Now raise the beam slightly to see whether the estimated 

 " weight " is nearly equal to the mass of the substance in E. If the weight 

 appears a little below what is wanted, pick up, with the forceps, the next 

 heaviest weight in the box, and try it in the pan with the other. If the 

 two together are too heavy, take out the smaller weight and put in the one 

 below it, and so on, adding one weight after the other, without missing any 

 until the correct weight is found. When you have completed the weighing, 

 write down and add up the weights that are missing from their places, and 

 check the figures as you put each weight back in its place. 



