Force and Osmotic Pressur 



39 L 



After a good many failures a form of " concentration " 

 cell was designed which satisfied the requirements of facility 

 of setting up and cleaning, freedom from diffusion, and low 

 internal resistance : it served equally for rod or amalgam 

 electrodes. A pair of glass tubes, A A fig. 2 (8 cm. x 2), 

 have each a short side tube, B B, blown on at about half 

 their length : the side tubes fit by means of short pieces of 

 rubber tubing inside a straight tube C (3 cm. x 1) which 

 carries the porous diaphragm ; this is a fragment of a porous 

 soup-plate D, filed round and fitted in by a ring of india- 

 rubber. Each tube A is closed by a rubber stopper with two 

 holes, one for the electrode ; the other, made merely for 

 convenience in filling, is eventually closed by a glass rod. 

 By working the side tube B backwards and forwards in the 

 central tube C, the air can be driven out of the latter. 



The cells were placed — up to six at a time — in a wooden 

 holder by which they were held in a sink filled with water; 



Flo-. 2. 



the sink was provided with a thermoregulator, and kept 

 within one or two tenths of 20°' 2 cent. The Clark cells were 

 kept in the same thermostat. 



Electrical measuring arrangements. — The E.M.F. of the 

 concentration-cells, or in the case of the u mercury " cells the 

 difference between their E.M.F.'s or between each one and a 

 Clark, was measured by the compensation method. At first 

 a Crompton potentiometer was used, but subsequently given 

 up on account of the necessity for frequently readjusting ; a 

 P.O. box was then adopted, the resistance being always kept 

 greater than 100 ohms. Under these circumstances the 

 accumulator (charged fortnightly, and discharged for half an 

 hour at maximum rate before using) gave such a constant 

 E.M.F. that it was only necessary to take a reading with the 

 Clark cell once a day to standardize. The connexions are 



