MEASUREMENT OF ELECTROMOTIVE FORCE. 9 



lyte. By means of this arrangement one such electrode lasted through a 

 whole season and at the end of that time gave no test for iron in the cell 

 itself. Thus the uncertainty of this possible error is removed and the 

 nuisance of repeated renewal of the electrode done away with. The arrange- 

 ment is shown on the left-hand side of figure I. 



The usual arrangement of the iron and of the cell was as shown in figure I, 

 and on a larger scale in figure 2. The sample, about half the size of a pea, 



Fig. 1. The Cell and its Appurtenances. 



A. Reservoir of decinormal potassic D. 



chloride. 



B. Reservoir of normal ferrous sul- E. 



phate containing pure iron F. 



wires. G. 



C. Calomel electrode (decinormal). 



Connecting receptacle containing 

 ferrous sulphate. 



Inverted U tube. 



Iron electrode (enlarged in fig. 2). 



Automatic hydrogen generator, pro- 

 tecting Reservoir B from air. 



was inserted in a running noose of platinum wire, the end of which projected 

 through a fine hole in a cork stopper. The noose was then pulled tight, 

 drawing the sample against the cork, and the wire was fastened by winding 

 around the shank of an iron tack on the other side of the cork. The cork 

 was fitted into one end of a glass tube and carefully sealed with molten 

 paraffin. The tube was filled with normal ferrous sulphate solution by 



