Relations between Electrical Measurements. 437 



we know the results of previous scientific inquiry, and are ac- 

 quainted with the method of adapting them to the case before 

 us, we may discover the proper arrangement at once. If we are 

 unable to make any estimate of what is required before construct- 

 ing the apparatus, we may have to encounter numerous failures 

 which might have been avoided if we had known how to make a 

 proper use of existing data. 



All exact knowledge is founded on the comparison of one 

 quantity with another. In many experimental researches con- 

 ducted by single individuals, the absolute values of those quan- 

 tities are of no importance; but whenever many persons are to 

 act together, it is necessary that they should have a common 

 understanding of the measures to be employed. The object of 

 the present treatise is to assist in attaining this common under- 

 standing as to electrical measurements. 



2. Derivation of Units from fundamental Standards. — Every 

 distinct kind of quantity requires a standard of its own, and 

 these standards might be chosen quite independently of each 

 other, and in many cases have been so chosen ; but it is pos- 

 sible to deduce all standards of quantity from the fundamental 

 standards adopted for length, time, and mass ; and it is of 

 great scientific and practical importance to deduce them from 

 these standards in a systematic manner. Thus it is easy to 

 understand what a square foot is when we know what a linear 

 foot is, or to find the number of cubic feet in a room from its 

 length, breadth, and height — because the foot, the square foot, 

 and the cubic foot are parts of the same system of units. But 

 the pint, gallon, &c, form another set of measures of volume, 

 which has been formed without reference to the system based 

 on length ; and in order to reduce the one set of numbers to the 

 other, we have to multiply by a troublesome fraction, difficult to 

 remember, and therefore a fruitful source of error. 



The varieties of weights and measures which formerly pre- 

 vailed in this country, when different measures were adopted 

 for different kinds of goods, may be taken as an example of the 

 principle of unsystematized standards, while the modern French 

 system, in which everything is derived from the elementary 

 standards, exhibits the simplicity of the systematic arrange- 

 ment. 



In the opinion of the most practical and the most scientific 

 men, a system in which every unit is derived from the primary 

 units with decimal subdivisions is the best whenever it can be 

 introduced. It is easily learnt; it renders calculations of all 

 kinds simpler; it is more readily accepted by the world at large; 

 and it bears the stamp of the authority, not of this or that legis- 

 lator or man of science, but of nature. 



