ELECTRICAL APPARATUS. 169 



in terms of the corresponding absolute units. Here, again, an 

 absolute standard having once been established, other resist- 

 ances can be measured in terms of it in various ways. The com- 

 parison of resistances is most frequently effected by means of some 

 form of the arrangement known as " Wheatstone's Bridge" (first 

 used by S. H. Christie, 1833 ; more fully described by Wheatstone, 

 1 843), the principle of which is, that the resistances of conductors, 

 traversed by the same current, are proportional to the differences 

 of potential between their extremities. For the purpose of such 

 comparisons, besides an original standard and trustworthy copies 

 of it, conductors are required having resistances which are definite 

 multiples or submultiples of that of the standard, and so arranged 

 that any one or more of them can be readily added to, or re- 

 moved from a conducting circuit. 



In connection with the methods that are available for the 

 measurement of the fundamental magnitudes dealt with in elec- 

 trical science, it is important to point out that, in some cases, a 

 given electrical magnitude say a quantity of electricity can be 

 measured by methods which differ not only as to the apparatus 

 employed and the nature of the observations made, but also in 

 respect of the physical principles upon which they are based. 

 Thus, in electrostatics, a quantity of electricity is estimated 

 (directly or indirectly) by the electric attraction or repulsion it can 

 exert at a given distance, and that quantity is taken as unity, 

 which exerts, upon another equal quantity at unit distance (one 

 centimetre), a force equal to unity (one dyne) ; whereas, in electro- 

 dynamics, the unit of electricity is the quantity which passes any 

 transverse section of a conductor when a current of unit strength 

 flows in the conductor for a unit of time (one second). The elec- 

 trodynamic measurement of a quantity of electricity thus depends 

 upon the measurement of the strength of a current, and this again 

 is based (directly or indirectly) upon the magnetic force exerted by 

 the current in its neighbourhood, the unit of current-strength being 

 the strength of a current which, if flowing for unit distance along the 



