330 MR. W. C. D. WHETHAM ON THE IONIZATION OF 



polarization effects than the telephone indicator, and it was hoped that, considering 

 the large area of the electrodes, platinization might be \innecessary. Measurements 

 were therefore made with the bright platinum surfaces. It was, however, found 

 that, while the results were satisfactory at extreme dilution, the resistances came 

 out higher than they ought as the concentration of the solutions increased. 



The cell was therefore platinized by passing a current from two accumulator cells 

 backwards and forwards between the electrodes through a solution of platinum 

 chloride containing a little lead acetate.* It then became very difficult to keep the 

 resistance of a dilute solution constant. It either slowly increased owing to 

 absorption of salt by the electrodes, or slowly diminished as the absorbed salt 

 was given out again to the solution. 



Mr. GRIFFITHS suggested that the surface obtained by heating platinum black 

 might answer the purpose required. When heated to redness, a platinized surface 

 becomes of a dull grey colour, and apparently loses its spongy texture. It gives, 

 however, a rough surface, the area of which is much greater than that of a bright 

 polished plate. The cell was dismounted, and the platinum portion heated for some 

 time to bright redness in a large blow-pipe flame. 



The surface thus obtained seemed to be quite satisfactory. The absorption effects 

 were apparently done away, and the area of the electrode was large enough to make 

 it possible to measure resistances as low as 10 ohms. Thus, in one apparatus, 

 resistances varying from 10 to 50,000 ohms were easily measurable to an 

 accuracy of one part in a thousand. 



SECTION 4. On the Measurement of the Electrical Resistance. 



Since very dilute solutions were to be examined in a platinum apparatus, the 

 method of direct currents applied successfully by STROUD and HENDERSON t to the 

 measurement of electrolytic resistances could not be adopted, and alternating currents 

 had to be used. The application of these by KOHLRAUSCH, OSTWALD, and others, a 

 telephone acting as indicator, is too well known to need description, but the method 

 adopted in these experiments, in which the telephone is replaced by a galvanometer, 

 is not so common. 



It was first described by FITZPATRICK,^ and the commutator used by him was 

 lent by the Cavendish Laboratory for the earlier part of the present work. An 

 instrument of improved design was afterwards employed, which is represented in 

 fig. 5. A vulcanite drum, revolving round a horizontal axis, has, near each end ot 

 its circumference, a series of brass strips fitted into it, alternate strips at each end 



* Lummer and Kurlbaum, 'Ann. der Physik und Chemie,' vol. 60, p. 315, 1897. 

 t 'Proc. Physical Soc. of London,' vol. 15, p. 13, 1896. 

 J 'Brit. Assoc. Report,' 1886, p. 328. 



