410 Mr. A. P. Laurie on Measurements of the 



ments made with this cadmium cell, for a special purpose. 

 The object which the investigator has in view in such re- 

 searches as these, is to determine the electromotive force 

 developed by a given set of chemical reactions ; and he there- 

 fore first devises a voltaic cell in which the given reactions 

 will occur on using the cell as a source of electricity, and 

 then measures its electromotive force. 



As examples of this kind of measurement it is only neces- 

 sary to refer to the papers by Dr. Alder Wright in the back 

 numbers of this Magazine. I know of no other set of experi- 

 ments approaching these in accuracy and completeness. 



These measurements, then, of electromotive force can be 

 made by two or three different methods : for instance, by 

 means of the quadrant-electrometer, or by means of a very 

 high-resistance galvanometer, with a few megohms in the 

 circuit. These methods of measurement, however, have one 

 point in common : an infinitely small current is drawn from 

 the cell which is being tested, whether it be to charge the 

 quadrants of the electrometer or to deflect the galvanometer- 

 needle. Unfortunately the use of these very small currents 

 leads one to doubt how far such measurements are reliable ; 

 for, be it remembered, it is not the measurement of the E.M.F. 

 of a given voltaic cell with an enormous resistance in the cir- 

 cuit which is required, but the measurement of the electro- 

 motive force due to a definite chemical reaction. 



Now an infinitely small current means an infinitely small 

 amount of chemical change ; and to take the cell at present 

 under consideration, how can we be sure that the source of 

 this current is the formation of cadmium iodide, and that it is 

 not due to impurities in the metal or the solution, or to a film 

 of gas or coat of oxide on the cadmium plate ? To this it may 

 be answered that experiments such as those of Dr. Wright 

 with a great variety of voltaic combinations, and the remark- 

 able agreement of the results one with another, give a con- 

 siderable confidence in such measurements. This is no doubt 

 correct, though it is still possible that constant sources of 

 error may occur. Then the approximate agreement of the 

 electromotive force found with that calculated from the 

 thermal data in so many cells confirms the measurements for 

 those cells at any rate. 



But in many cases Dr. Wright has found that not only do 

 the calculated and measured electromotive forces differ con- 

 siderably, but in several voltaic combinations the current 

 actually flows the opposite way to that calculated from the 

 thermal data. 



Such a result, if proved, is of the greatest interest ; but I 



