Electricity in a uniform plane conducting Surface. 477 



several points arbitrarily chosen, and in each case finding, by trial 

 with a wire from the other terminal, a sufficient number of points 

 having the same potential as the selected point. To ascertain 

 the distribution required to make the difference of potential be- 

 tween consecutive lines constant, he determined a number of 

 points along the straight line joining the electrodes, such that the 

 difference of potential between each one and the next was just 

 sufficient to compensate the constant electromotive force of a 

 thermoelectric element. The form of the equipotential lines in 

 some rather more complicated cases was determined in the same 

 manner by Quincke*, and in other cases again by students in the 

 Edinburgh University Laboratory j\ SchwedoffJ also has in- 

 vestigated the direction of the flow at various points in a rectan- 

 gular sheet of tinfoil. 



Recently an elaborate series of experiments, by a method 

 similar to KirchhofFs, has been carried out by Professor W. Gr. 

 Adams §, who has also extended the same method of inquiry to 

 conductors of three dimensions by the employment of conducting 

 liquids (mercury, sulphate of copper, sulphate of zinc). 



Verifications by Kirchhoff and by Mach of the theoretical ex- 

 pressions for the strength of the current at different parts of a 

 conducting sheet have already been referred to (see Part I. § 15, 

 footnote, Phil. Mag. S. 4. vol.xlix. p. 395). 



Experimental measurements of resistance, in addition to the 

 attempts of Kirchhoff already mentioned (Parti. § 1), have been 

 made by Gaugain||, who measured the resistance of a solution of 

 sulphate of copper contained between two circular copper cylin- 

 ders placed one inside the other with their axes parallel, and 

 thus verified Blavier's formula (see Part I. § 24, footnote) — by v. 

 Obermayr^l, who verified Kirchhoff 's formula for circular disks, 

 and also a formula deduced by Stefan for the resistance of an 

 oblong strip with circular electrodes placed on the central line, 

 by experiments with disks and strips of platinum-foil — and by 

 Domalip**, who measured the resistance of thin circular layers 

 of a solution of sulphate of zinc contained between parallel glass 



* Pogg. Ann. (1856) vol. xcvii. p. 382. 



t W. Robertson Smith, Proc. Edinb. Roy. Soc. 1869-70, p. .96. 



X Schwedoff (Pogg. Ann. 1873, Erganzungsb. vi. p. 85) inferred the 

 direction of the resultant flow at any point of a conducting sheet in which 

 there are two equal opposite poles, from simple geometrical considerations, 

 to a great extent similar to those employed by Professor W. R. Smith and 

 by ourselves (Phil. Mag. vol. xlix. p. 394). We should have referred to his 

 paper before, if we had known of its existence. 



§ Proc. Roy. Soc. (1875) vol. xxiii. p. 280. 



|| Ann. de Chim. et de Phys. [3] vol. lxiv. p. 200 (1862). 



% Wien. Akad. Ber. (1869) vol. lx. pt. 2, p. 245. 



** Carl's Repertoriumfur Experiment al-Physik, 1874, vol. x. p. 23. 



