EXPERIMENT STATION BULLETINS. 387 



The apparatus required for studying these changes is that ordinarily 

 used in determining the electrical conductivity of solutions. It consists 

 essentially of the following: 



1. Electrolytic cell. 



2. An accurate resistance box. 



3. Wheatstone bridge. 



4. Induction coil. 



5. Telephone receiver. 



G. Source of electromotive force, i. e., a small storage cell or dry 

 battery. 



7. Telegraph key or switch. 



8. Large tank or tub of water, preferably with constant temperature 

 attachment. 



There are several kinds of electrolytic cells. They consist essentially 

 of two electrodes of platinum foil to which are fastened platinum wires 

 sealed into glass tubes. The glass tubes may be filled with mercury 

 and the contact made by dipping the ends of the connecting wires into 

 them. The authors used the so-called "Oswald conductivity cell," which 

 gave very good results. Hall and BelP recommended the use of the ''H. 

 C. Jones cell for concentrated solutions." 



The resistance box may be of the ordinary decade type but should have 

 an accuracy of .1%. 



The wheatstone bridge in its simplest fonn consists of a long wire 

 of resistance alloy connected to terminals at both ends and fitted with 

 a sliding contact. It is mounted on a scale, usually an ordinary meter 

 stick. The best instruments now have this wire coiled on a marble 

 cylinder. They are of two types; one where the cylinder and wire are 

 stationary while the arm holding the sliding contact moves, and the other 

 where the ann is stationary and the cylinder and wire move. Either is 

 suitable for the purpose. 



Any small induction coil giving a regular high pitched vibration will 

 do. One such as is used in ordinary medical batteries is quite satisfac- 

 tory. 



The storage cell used by the authors was one made for use on auto- 

 mobiles. Dry cells may be used in place of it. 



The fact that electrical resistance varies greatly with temperature 

 makes a thermostat of some sort imperative. Hall has shown that a 

 large tub of water can be used to good advantage, but of course 

 wherever possible a regular thermostat with a constant temperature regu- 

 lator should be used. 



Before proceeding with the actual determination, the platinum elec- 

 trodes of the cell must be coated with platinum black. The manner of 

 doing this is described by Findlay.^ The glass cell is first steamed for 

 about an hour to remove soluble salts, after which a solution of 3 grams 

 of platinum chloride and 0.02 to 0.03 grams lead acetate per 100 c. c. of 

 water is placed in it. After cleaning the electrodes with chromic acid, 

 they are placed in this solution and connected with a battery and such 

 a resistance inserted in the circuit that the amount of gas evolved is not 

 too rapid. The current should be passed for 20 to 30 minutes, the wires 

 dipping into the tubes of mercury being interchanged every half minute. 



>Loc. Cit. 



'Practical Physical' Chemistry, Longmans, Green''& Co., 1906. P.^150. 



