436 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



terms of the displacements of the points of application of the forces 

 acting on its surface, on the supposition that the passive forces are 

 replaced by any active forces which can be in equilibrium with the 

 applied forces, then, since 



W=2{Xdu+YZv + ZZw}, 



where u, v, w are the component displacements of the point of appli- 

 cation of the force whose components are X, Y, Z, we have 



^=X —=Y —=Z 



du dv ' dw 



equations which express a property of U analogous to the well-known 

 property of the potential energy of a particle consequent on its vici- 

 nity to an attracting body, and to the properties of the accumulation 

 of vis viva of a dynamical system. 



LXI. On the Elementary Relations between Electrical Measure- 

 ments. By Professor J. Clerk Maxwell and Pleeming 

 Jenkin, Esq.* 



•Part I. — Introductory. 



1. REJECTS of Treatise. — The progressive extension, of the 

 ^ electric telegraph has made a practical knowledge 

 of electric and magnetic phenomena necessary to a large num- 

 ber of persons who are more or less occupied in the construc- 

 tion and working of the lines, and interesting to many others 

 who are unwilling to be ignorant of the use of the network of 

 wires which surrounds them. The discoveries of Volta and 

 Galvani, of (Ersted, and of Paraday are familiar in the mouths 

 of all who talk of science, while the results of those discoveries 

 are the foundation of branches of industry conducted by many 

 who have perhaps never heard of those illustrious names. Between 

 the student's mere knowledge of the history of discovery and the 

 workman's practical familiarity with particular operations which 

 can only be communicated to others by direct imitation, we are 

 in want of a set of rules, or rather principles, by which the laws 

 remembered in their abstract form can be applied to estimate 

 the forces required to effect any given practical result. 



We may be called on to construct electrical apparatus for a 

 particular purpose. In order to know how many cells are 

 required for the battery, and of what size they should be, we 

 require to know the strength of the current required, the elec- 

 tromotive force of the cells, and the resistance of the circuit. If 



* Reprinted from the Reports of the British Association for 1863. 



