1 62 SCIENTIFIC APPARATUS. 



traversed by a current of electricity. If by any means the differ- 

 ence of potential between A and B can be maintained constant, 

 notwithstanding their being connected by another conductor, this 

 conductor exhibits permanently the special properties which, in 

 the case first supposed, it exhibited only momentarily. In this 

 case a condition of dynamical equilibrium is maintained, as long 

 as the conditions remain unchanged, which is expressed by saying 

 that the conductor conveys a constant electric current, or that an 

 equal quantity of electricity passes any transverse section of it in 

 each unit of time. 



The first conditions requisite for the production of any electro- 

 dynamical phenomenon are therefore the existence and main- 

 tenance of a difference of potential between two points ; and, 

 secondly, a conductor connecting these points in which the dif- 

 ference of potential can give rise to an electric current. 



The apparatus which is generally most convenient for the former 

 purpose is some form of voltaic battery, thermo-electric battery, 

 or, when a rapid succession of currents of short duration or 

 of currents in opposite directions is admissible, magneto-electric 

 machine, or induction-coil. Ordinary electrical machines , may be 

 used with advantage in special cases, but, although capable of pro- 

 ducing great differences of potential between insulated conductors, 

 the difference of potential which can be maintained by means of 

 them between the extremities of a good conductor is much less than 

 that which can be kept up by the instruments previously named. 



The general construction of the voltaic battery is well known. 

 Of the countless modifications that have been introduced, 

 or at least proposed, none, since Daniell (1836) showed how 

 to make "constant" batteries, are of a fundamental kind; 

 several considerable improvements, however, have beeji made in 

 recent years in matters affecting the practical convenience of 

 voltaic batteries and their adaptation to special purposes. Similar 

 remarks are applicable also to recent modifications in the con- 

 struction of thermo-electric batteries. 



