425 



formation of sulphate of zinc dissolved, (the electro-motive 

 force from the latter is generally supposed to be extremely 

 small,) divided by the resistance to conduction of the fluid, 

 (in technical language, the electrolyte,) and also of the 

 plates themselves, and the conducting wire connecting them. 

 I have selected this form of battery as visibly manifesting 

 these effects, for, on connecting the plates by a short wire, 

 the force F is considerable, and is manifested by the energy 

 of the evolution of gas ; but if I introduce a greater resist- 

 ance into the circuit, the evolution of gas will be much 

 diminished : by introducing the galvanometer into the circuit 

 we obtain an estimate of the force in grain weights sup- 

 ported. If we add a still greater length of wire, for 

 instance, adding a coil of 70 feet, we shall find the evolution 

 of gas again diminished, and in like manner the force as 

 manifested by the diminution of weight supported. 



So far the theory is beautiful and unexceptionable; but 

 in further applications of the theory as stated by Professor 

 Wheatstone, " the resistance of the connecting wire of the 

 circuit is directly proportional to its length, and to its 

 specific resistance, and inversely proportional to its section." 



Some experiments were made by Mr. Barlow many years 

 ago, but which were imperfect, both from the construction 

 of the batteries employed, and from not taking into account 

 the resistances of the battery itself, (Ohm's theory being 

 then little known.) From these experiments it appeared 

 that the resistance was directly proportional to the square 

 root of the length of the wire, and inversely proportional 

 to the quantity of metal contained in the wire. 



Some experiments confirm these views, particularly when 

 the resistances of the battery are not estimated. The 

 results of my own experiments, which are not yet so com- 

 plete as I desire, lead me to suppose that the same law of 

 conduction does not hold in regard to currents of consider- 



