ON STANDARDS OF ELECTRICAL RESISTANCE. 131 
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 conducted by single individuals, 
the absolute values of those quantities 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 understanding 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 possible 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 prevailed 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 arrangement. 
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 subdivi- 
sions is the best whenever it can be introduced. It is easily learnt; it 
renders calculation 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 
legislator or man of science, but of nature. 
The phenomena by which electricity is known to us are of a mechanical 
kind, and therefore they must be measured by mechanical units or standards. 
Our task is to explain how these units may be derived from the elementary 
ones ; in other words, we shall endeavour to show how all electric phenomena 
may be measured in terms of time, mass, and space only, referring briefly in 
each case to a practical method of effecting the observation. 
3. Standard Mechanical Units——In this country the standard of length is 
one yard, but a foot is the unit popularly adopted. In France it is the ten 
millionth part of the distance from the pole to the equator, measured along 
the earth’s surface, according to the calculations of Delambre, and this mea- 
sure is called a metre, and is equal to 3:280899 feet, or 39°37079 inches. 
The standard unit of time in all civilized countries is deduced from the time 
of rotation of the earth about its axis. The sidereal day, or the true period 
of rotation of the earth, can be ascertained with great exactness by the ordi- 
nary observations of astronomers; and the mean solar day can be deduced 
from this by our knowledge of the length of the year. The unit of time 
adopted in all physical researches is one second of mean solar time. 
The standard unit of mass is in this country the ayoirdupois pound, as we 
received it from our ancestors. The grain is one 7000th of a pound. In the 
K 2 
