fur Voltaic Electricihj, ^c. 121 



3, &c. feet of wire which is introduced into the circuit for expe- 

 rimental purposes. It has to pass through the galvanometer 

 wire, the nitric acid, the porous cell, the sulphuric acid, &c. We 

 must therefore in our calculations suppose an addition of a length 

 of wire to that which is used in the experiments, and which 

 remains constant for one series of experiments. Thus suppose 

 E the electromotive power of the battery, R the resistance of 

 battery expressed by the length of wire (of the same kind as that 

 used in the experiment), which would offer the same resistance 

 as the fluids and solids of the battery actually do offer to the 

 passage of the current. Let a; denote the length of wire intro- 

 duced into the circuit for the sake of experiment. Then we ex- 

 press the force of the current by E-r-(R + A), and not by E-h^, 

 as has been very commonly and very erroneously done. From 

 two observations, the values of E and R (which may approxi- 

 mately be supposed constant for one series of experiments) may 

 be found ; and then by giving to x the values 1, 2, 3 . . . suc- 

 cessivelj', we may obtain corresponding calculated deflections of 

 the galvanometer which may be compared with the results of 

 experiments. I have thus calculated all the four tables of expe- 

 riments, and placed the results of theory and experiment side 

 by side. The number of observations recorded in the last three 

 tables is so small, that they are not of much real importance as 

 tests. 



As Mr. Dresser thought that he found the greatest deviation 

 from the commonly received law when he experimented with 

 short lengths of wires, I am led to the conclusion that he em- 

 ployed the formula Eh- a; in his calculations. This would amount 

 to a supposition, that the current had no other resistances to 

 overcome beyond that arising from its passage through the wire 

 introduced into the circuit, as had previously been done by Pro- 

 fessor Barlow, I believe, and others. The absurdity of such a 

 supposition will be seen by a simple illustration. Suppose that 

 a person wished to find experimentally the law of resistance to a 

 carriage carrying various numbers of passengers when propelled 

 on a railway at a given velocity. Suppose that the tractive 

 power required for 1, 2, 3 . . . 20 passengers of equal weights 

 was measured with great nicety, still \ery little confidence would 

 be placed in any general law connecting the number of passen- 

 gers and the tractive power when the heavy weight of the car- 

 riage itself was entirely neglected. 



