WEIGHTS AND MEASURES 29 



substance. After a balance is obtained known weights 

 are substituted for the unknown body until a balance 

 is again obtained. It is evident that these known 

 weights produce the same effect under the same con- 

 ditions as does the body of unknown weight and hence 

 that they are equivalent. The method requires that 

 everything else which can hi any way affect the actions 

 to be compared must remain unchanged, or in the 

 usual words, all other factors must be constant. 



We are now ready to give a name to the method of 

 direct comparison, illustrated by the use of the balance. 

 If the lever arms are equal, equal weights produce 

 equal and opposite effects which neutralize each 

 other, so that there is no deflection of the beam. This 

 is therefore called the " opposition method," the "zero 

 deflection method," or the "null method" from the 

 Latin "nullus." It is the method of no resultant 

 effect. 



These two methods, the opposition and the sub- 

 stitution, are the only ones by which we can compare 

 two things, whether weights, electric currents, light 

 intensities, or any of the other magnitudes which the 

 scientist may have occasion to measure. The method 

 of double weighing is evidently merely the opposition 

 method used twice with the weights reversed, so that 

 the effect of inequalities in the lever arms of the balance 

 may be eliminated. 



The advantages of the two methods are now evident. 

 With the opposition method a direct comparison is 

 obtained by one operation, while with the substitution 

 method two operations are required. On the other 

 hand, if the possible inequalities in the two parts of the 



