1 68 SCIENTIFIC APPARATUS. 



It will be observed that this relation is exactly parallel to that 

 stated above (p. 159) between the quantity of an electric charge, 

 and the potential and capacity of an insulated conductor, and, just 

 as in that case, so here, a measurement of any two of the three 

 /elated qualities gives indirectly the value of the third. The 

 above relation, however, taken alone, would afford only compara- 

 tive measurements of the quantities involved ; but by combining 

 it with that which connects the work done by a current in a con- 

 ductor with the electrical conditions of the conductor, namely 

 the work done in tmit of time between any two points of an 



electric circuit =. strength of the current x difference of potential 



betiveen the given points, 



the three electrical magnitudes, strength of current, difference of 

 potential, and conducting power, can be expressed in terms of the 

 units employed for the measurement of work, namely, the units 

 of length, mass, and time, and are then said to be expressed in 

 absolute measure. The introduction of absolute measures into 

 electrical science is due to W. Weber. 



The principle of the methods available for the direct measure- 

 ment of currents has been already pointed out. An absolute 

 standard for the measurement of difference of potential may be 

 founded on the principle that, when (the whole or any part of) a 

 conducting circuit is moved, in such a way as to cause an altera- 

 tion in the magnetic force acting through the area bounded by the 

 circuit, an "electromotive force" equivalent to a difference of 

 potential, is generated, whose value at any instant is equal to 

 the rate at which the magnetic force through the area of the 

 circuit is then varying. A difference of potential produced 

 in any other way may be compared with such a standard by 

 various methods, of which those most generally applicable are 

 adaptations of a method given by Poggendorff, in 1841. An 

 absolute measure of the resistance of a conductor is given by 

 the ratio of the difference of potential (or electromotive force) 

 acting in it to the current produced, this ratio being expressed 



