Papers on Contact Electricity. 51 



or about what Exner found it for a dilute sulphuric-acid 

 Smee's cell*. 



Secondly, as regards the variations of the electromotive 

 force of a Smee's cell, Sir Wm. Thomson calculated! from Dr. 

 Andrews's experiments that, if we consider the oxidation of the 

 zinc and the combination of this oxide with strong sulphuric 

 acid, minus the energy necessary to decompose the equivalent 

 quantity of water, we obtain as the electromotive force of a 

 Smee's cell 2,056,200 British absolute units, which corresponds 

 with 0*82 Daniel! [with more modern data Exner gives 0*73], 

 since the electromotive force of a Daniell's cell as determined 

 experimentally by Joule is 2,507,100 British absolute units. 

 Sir William adds : — " It is to be remarked that the external 

 electromotive force determined for a single cell of Smee accord- 

 ing to the preceding principles, by subtracting the chemical 

 resistance " [this latter being due to the evolution of hydrogen 

 upon the platinized silver] " from the value J6e, is the perma- 

 nent working external electromotive force. The electrosta- 

 tical tension which will determine the initial working external 

 electromotive force depends on the primitive state of the plati- 

 nized silver plate. It could never be greater than to make 

 the initial working force J x 1670x6, or 5,444,500 " [2'17 

 Daniells according to the above reduction], " corresponding to 

 the combination of zinc with gaseous oxygen, and of the oxide 

 with sulphuric acid. It might possibly reach this limit if the 

 platinized surface had been carefully cleaned and kept in oxy- 

 gen gas until the moment of immersion, or if it had been used 

 at the positive electrode of an apparatus for decomposing 



absorbed, by the liquid &c, and will always be noticed if the measuring- 

 apparatus be only sufficiently delicate. Such differences have nothing in 

 common with the great discrepancies referred to at the commencement of 

 this note, on the results obtained by Prof. Exner and by other experimen- 

 ters for the electromotive force at the contact of metals, since the latter is 

 fairly constant at a constant temperature, so that any great discrepancies 

 in such measurements can only arise from errors in experimenting. 



* We are unable, from our results at present published, to calculate 

 exactly the electromotive force of Smee's dilute-acid cell, since, as 

 regards liquids, it was only with distilled water and with strong acids 

 that the electromotive force of contact of platinum was measured in our 

 experiments. We can, however, from these latter approximate to limiting 

 values of the electromotive force of a Smee's dilute-acid cell; that is, 

 using the values of the contact electromotive forces of platinum with water 

 and with strong sulphuric acid, and assuming that the contact of platinum 

 with dilute sulphuric acid is somewhere between these two, it follows 

 that the electromotive force of a Smee's dilute-acid cell, as determined 

 from the sum of the separate contact differences of potential, must at 

 first be greater than T567 volt. 



t " On the Application of Mechanical Effect to the Measurement of 

 Electromotive Forces," by Prof. Thomson, Phil. Mag. Dec. 1851. 



E 2 



