256 THE ELECTRIC TELEGRAPH. 



say, about 64 of Siemens' units ; that in the second space has 

 a resistance representing two such miles, and so on to the 

 sixth space, the wire of which has a resistance of six miles. 

 Any two neighbouring rings can be brought into short-circuit 

 by means of the brass contact pieces which turn on pins in 

 the upper rings. In this way either or all of the coils may 

 be short-circuited the current passing only through those 

 which are open. When all the brass contact pieces are closed 

 the resistances are all short-circuited ; when all are open the 

 resistance between the top and bottom rings is equal to 

 1 + 2 + 3 + 4 + 5 + 6=21 miles. 



28. Ohm's Law. Until the end of the first quarter of the 

 present century physicists were still in darkness as to the 

 mode and laws of the propagation of the galvanic current. 

 The immense velocity with which the galvanic impulse is 

 transmitted led to the seeking an analogy between it and 

 light ; and on this wrong scent much time and labour were 

 lost, when Ohm, a German physicist, conceived the happy 

 idea that a juster analogy was to be found in the propagation 

 of heat, and proceeded to apply to galvanic electricity the 

 formulae of Fourrier and Poisson. He expressed the intensity 

 of an electric current as directly proportional to the electro- 

 motive force, and inversely to the resistance of the circuit. 

 Algebraically, if E is the electro-motive force, R the resistance, 

 and I the intensity, 



Of these magnitudes R is made up of two resistances that 

 interior and that exterior to the element. The internal 

 resistance, or resistance of the element, is, again, the sum of 

 the several resistances due to the passage of the current from 

 one plate to the liquid, to its passage through the liquid, and 

 to its passage from the liquid to the other plate. We will 

 call this resistance of the element, r. The remaining com- 

 ponent the external resistance is that due to the passage 

 of the current through the interiors of the plates, the wire 

 connecting them, and through whatever conductor may be 



