128 Dr. J. A. Fleming on the Use ofDanielVs Cell 



the board is fastened a broad copper strap b b, having six ter- 

 minals fastened upon it. Between the last of these terminals 

 and the end of the scale-wire A / is inserted a reflecting-galva- 

 nometer of 5000 ohms resistance, and an additional resistance 

 of 50,000 ohms R. 



To the other terminals s, s on this copper strip are con- 

 nected one pole (like to the pole of the battery B connected 

 to M') of each of the cells c, c f to be compared ; and the 

 other poles of these cells are connected with sliders S, S / 

 travelling over the wires. These sliders are blocks of wood 

 sliding in the groove between the scales, and overhanging 

 the wires. On them are German-silver spring-strips as shown 

 in the figure, and which, when pressed down, make contact 

 with the wire. The strips are backed with leather to avoid the 

 production of thermoelectric currents. By using two or more 

 of these sliders, it is easily seen that several cells may be 

 balanced at once on the potentiometer; and in particular two 

 cells c and d can be compared in respect of electromotive 

 force very accurately, even although the E.M.F. of the main 

 battery is not quite constant. The introduction of the resist- 

 ance 11 prevents any but the very smallest currents passing 

 in the cells when the place of balance is being found on the 

 potentiometer. The German-silver strips g, g on the sliders 

 make contact only when pressed down; so that in the normal 

 condition the cells c, c' are insulated. With the galvanometer 

 in a sensitive condition it is very easy to read a difference of 



Toooo °f a vo ^ on ^° Wlre ) an( i toW can b° reac ^ with great 

 accuracy. 



Electromotive forces are read off directly as lengths, since 

 the E.M.F. of the cells c, c' is directly as the distance of the 

 constant points of their respective sliders from the end A' of 

 the wire. Great care has to be taken in the first instance to 

 stretch the wire uniformly, and to calibrate it if it presents 

 any want of uniformity of resistance. Provided with this 

 convenient means of directly comparing electromotive forces, 

 attention was next directed to a modification of Daniell's cell 

 suitable for the experiments required. In Dr. Alder Wright's 

 extensive researches on the subject*, the form of cell employed 

 was that sometimes called Kaoult's form, in which two small 

 beakers contain the two solutions, and are connected by an 

 inverted Y-tube, the ends of which are tied over with thin 



* u On the Determination of Chemical Affinity in Terms of Electro- 

 motive Force. — Part V. On the Relationships between the Electro- 

 motive Force of a Daniell ("ell and the Chemical Affinities involved in its 

 Action." Proceedings of the Physical Society of London, Vol. V. Part I. 

 p. 4 1 et srq. [Phil. Mag, vol. xiii. p. 265.] 



