36 MEASUREMENTS 



Mass is a constant property of a body, while weight will vary from place 

 to place as the acceleration of gravity varies. If we say that an object 

 "weighs" 15 g, we really are referring to its mass. 



The standard unit of mass is the international kilogram, a cylinder 

 of platinum-iridium kept in the vault in Paris. It was originally intended 

 to be equal to the mass of 1000 cm^ of water at 4° C, but later more 

 precise measurements have shown that this is not exactly correct. Thus 

 the kilogram, like the "old" meter, becomes an arbitrary standard. Since 

 July 1, 1959, the international pound is defined as 0.45359237 kg. 



Multiples larger than the kilogram are rarely used in the laboratory, 

 and, in fact, the gram (10^^ kg) is probably the most commonly used 

 unit. The prefixes and symbols of Table 4-1 apply quite directly to mass 

 units. The microgram has sometimes been given the symbol gamma (y), 

 but this, like the use of A. for (A, is being discouraged. 



Measurements of mass are perhaps the easiest of all measurements to 

 make. It is unfortunate that we speak of "weighing" for the comparison 

 of unknown masses with standard masses. We now can use a variety of 

 balances operating on slightly different principles. The simplest idea, 

 probably, is to place the unknown and standard masses on the opposite 

 and equal arms of a beam. Standard weights are added until there is no 

 deflection of the beam. The usual analytical balance is of this type. 

 Routine weighings of larger objects might be performed on a triple beam 

 balance, where a set of standards counteracts the unknown mass, not by 

 changing the amount of standard mass, but by changing the distance 

 between the standard and the point at which the beam is suspended. 

 From the simple law of the lever the scale can be graduated in mass units. 

 The "trip scale" is a combination of the previous two types. The material 

 to be weighed is placed on the left pan, known weights are placed on 

 the right pan to the nearest gram, and then the fractions of grams are 

 found by sliding a "rider" weight along a beam. 



A torsion balance contains a wire or band of metal which is twisted 

 during the measurement. Within the range of the balance, the amount 

 of twisting is proportional to the load. Most torsion balances are used as 

 null instruments, being brought back to the undeflected position by 

 adding weights to oppose the load or by moving riders. Several brands 

 return the balance to the undeflected position by moving an arm which 

 twists the wire in the opposite direction. The arm moves over a scale 

 from which the unknown mass can be read. 



Several new analytical balances are now being made that may even- 

 tually replace the equal-arm analytical balance. One difficulty with plac- 



