300 



Messrs. L. Mond and C. Langer. 



[June 20, 



set of plates is now coated on the four sides not covered by the zinc 

 plates with a mixture of beeswax and resin, so as to obtain a block 

 which is perfectly gastight all round, and the space V left between 

 the two zinc plates is then filled up with plaster of Paris, so as to 

 obtain one solid compact block with no openings except the entrances 

 and exits for the gases. This is closed by four more zinc plates, Z 3 , 

 Z 4 , Z 5 , Z 6 , which are soldered together so as to form a box. 



The E.M.F. of this battery we found to vary considerably, accord- 

 ing to the way in which the platinum black is prepared. The best 

 and most regular results we have obtained from platinum black made 

 by neutralising a boiling solution of PtCl 4 with ]Sra 2 C0 3 , and adding 

 this slowly to a boiling solution of sodium formiate. With this we 

 obtain an electromotive force of 0'97 volt with the open circuit. The 

 internal resistance varies considerably with the thickness of the 

 porous plates, the amount of the electrolyte contained in these, and the 

 surface of these plates. Plates of gypsum of 8 mm. thickness and 

 350 cm. surface gave an internal resistance of 0"02 ohm. 



The current obtainable from these batteries varies necessarily with 

 the external resistance. It is possible to obtain 8 amperes from one 

 such element, but the E.M.F. of the battery sinks at a very much 

 more rapid rate than with constant batteries if a strong current is 

 taken out, and the work done by the battery is not, as in constant 

 batteries, at its maximum when the internal and external resistance 

 are equal. Thus we found with a small battery of 42 sq. cm. surface 

 and 0*36 ohm internal resistance the following results : — 



E. 



: 





P. 



S. 



E. 





D. 



A. 



0-360 



0-30 







265 



0-883 



0-62 



0-58 



0-04 







•236 



>} 



0-40 







•310 



0-775 



0-62 



0-58 



0'04 







•240 





0-70 







•430 



0-614 



0-67 



0-65 



0-02 







•263 



j> 



1-00 







•490 



0-490 



0-68 



0-66 



0-02 







240 



j? 



2-00 







605 



0-302 



0-72 



0-71 



o-oi 







219 





4-00 







•700 



0-175 



0-77 



0-76 



o-oi 







122 



5) 



8-00 







•780 



0-097 



0-81 



0-81 



o-oo 







075 



33 



20-00 







•860 



0-043 



0-87 



0-87 



o-oo 







036 



Ei. Internal resistance in ohms. 



E.J. External resistance in ohms. 



P. Difference of potential at the poles. 



S. Strength of current in amperes. 



E. E.M.E. in volts, determined by the first deflection of the galvanometer needle 



1 second after opening the circuit. 

 Ej. Calculated E.M.E. in volts. 



D. Difference of E and Ei = increase of E.M.F. during 1 second, showing 

 rapidity of absorption of the gases with varying saturation of the platinum. 

 A. Work done by the battery in watts. 



