328 KEPORT— 1886. 



A number of speculations are suggested by the relations between the numbers 

 thus arrived at ; but it is not necessary to discuss them here. (See ' Phil. Mag/ 

 Feb. 1887.) 



On the application of Alternating Currents to the Determination of the 

 Conductivity of Electrolytes. By T. C. Fitzpatkick, Scholar of Christ's 

 College, Camiridge. (Communicated hy "W. N. Shaw.) 



The work described in this paper was undertaken with the view of testing the- 

 method of the alternate current as appUed to the measurement of tlie resistance- 

 and conductivity of electrolytes. 



This method was first employed by Kohlrausch and Nippoldt in 1869, and an 

 account of their work and their justification of the method is found in Poggendorff's 

 ' Annalen,' cxxxvi. They found that above a certain rate of alternation of the^ 

 current the electrolytic cell could be replaced by a metallic resistance, and that the 

 value of this replaced resistance did not alter on increasing the rate of alternation. 



Two things they considered were necessary for the complete removal of the 

 polarisation effects : — 



(ft) A sufficient rate of alternation. 



(b) Electrodes of considerable area ; small platinised platinum electrodes wer& 

 afterwards found to answer perfectly well. 



The alternate current was produced by a sine inductor, and as indicator they 

 employed a dynamometer. 



In the October number of the ' Annalen ' for last year there is a long paper on 

 Kohlrausch's most recent work ; the method is the same as that employed in the 

 earlier worJi, only the dynamometer is replaced by a telephone as indicator. 



A modification of this method was used by Ewing and Macgregor. They em- 

 ployed the Wheatstone bridge arrangement, the current being alternated by a 

 rocker. They do not, however, appear to have obtained any satisfactory results.^ 



More recently Macgregor has replaced the rocker by a double commutator. By 

 one half of the commutator the alternate current has been produced and by the 

 other the current of the galvanometer circuit has been redirected, so that through 

 the galvanometer there is a direct but intermittent current. It is a method similar 

 to this latter that I have employed. 



My commutator, which was very carefully made by the Camln-idge Scientific 

 Instrument Company, consists of a drum, on the two faces of which are eight 

 sectors of brass separated by strips of ebonite ; the alternate sectors are con- 

 nected with two brass rings on the spindle of the commutator. The only difference 

 between the two faces of the drum is that on the one side the sectors are smaller, 

 and the ebonite pieces bigger. This is to permit of the galvanometer circuit being 

 broken before the battery circuit, and made after it. 



The two rings on the one side of the commutator are connected by brushes 

 with the poles of the battery, and two brushes carry off' alternate currents from the 

 face of the drum to the two points on the Wheatstone bridge. The brushes are 

 kept firmly pressed in contact by springs. 



The two points in the bridge for the galvanometer are connected with two 



' Note ly Professor J. A. Eiving, in a letter to the Editor. — ' This reference is not 

 quite accurate. The method we used was not properly an alternate-current method. 

 Tlie plates were allowed to depolarise first.then the circuit (a Wheatstone-bridge one) 

 was completed — i.e , the battery key was pressed, the galvanometer key already being 

 down, and the initial impulse of the galvanometer (a very light narrow one) was 

 noted. The bridge was adjusted till this initial impulse was zero, as well as could 

 be judged. Of course this plan is open to the self-induction objection to which 

 alternate-current methods are liable ; and I should not recommend it now. As a 

 matter of fact, however, it gave much more consistent and apparently better results 

 than one might expect. Some of these, on the resistance of mixtures (solution of 

 sulphate of zinc mixed with sulphate of copper), were rather interesting The paper 

 was published in Trans. B.S.E., vol. xxvii., 1873. It was rather a boyish performance. 

 Still, if reference is to be made to it, it may as well be made correctly. — J. A. E.' 



