52 Domestic Science 



when the balance is not in use. From one end of the 

 beam in some forms of balance from each end projects 

 a thin brass rod with a fine screw-thread upon its 

 surface. A milled nut travels along this rod. Such 

 a balance as has been described is usually kept within 

 a wooden case with glass sides, in order to protect its 

 parts from the action of damp air and to keep them as 

 free as possible from dust. 



The balance is used to determine equality of masses. 

 Any body placed in one pan is attracted downwards 

 by gravity with a force proportional to its mass. If 

 another body or bodies be placed in the other pan and 

 it be found that the beam of the balance comes to rest 

 in a horizontal position, we know, since the arms of the 

 beam of the balance are of equal length, that the force 

 with which the contents of the second pan is attracted 

 downwards is equal to that acting on the first body, 

 in accordance with the Principle of Moments. Since 

 these forces the weights of the respective bodies- 

 are equal, we infer that the masses of the bodies in 

 the pans are also equal. 



30. In order to simplify measurements of weight, 

 units of weight have been adopted, just as is the case 

 of the other classes of measurements previously dealt 

 with. The British standard unit, the "pound", may 

 be defined as " the weight of a piece of platinum, 

 weighed in a vacuum at the temperature of melting 

 ice, which is kept in the Standards Department of the 

 Board of Trade". The need for specifying the exact 

 conditions under which the weighing must take place 

 will be grasped when the chapters dealing with dis- 

 placement of fluids and the effect of heat on bodies 

 have been read. Multiples and submultiples of the 

 standard unit are given in the subjoined table. 



