Platina Electrodes of a Voltaic Battery. 381 



the battery, the plate A to the positive and B to the negative 

 for a few minutes ; the connexions being broken, the zinc rod 

 was immersed as in the last experiment, its wires dipping into 

 the cups A and B ; iiydrogen was again evolved and collected 

 in the tubes. 



In that over the plate A was found 2*1 



B 1-0 



These experiments clearly pointed out, that the cause of 

 the difference in volume of evolved hydrogen was to be sought 

 in some change produced by connexion with the battery; 

 what that change consisted in was less obvious. I suspected 

 that a polar state might probably have been communicated to 

 the electrodes by the passage of the battery current. 



Exp. 4. The cups A and B were again connected with the 

 battery, A with the negative, B with tlie positive wire for a 

 few minutes ; the connexions were then broken, and a galvano- 

 meter of very delicate construction connected with the cups. 

 The needles instantly deviated with considerable velocity to 

 90° ; the connexion was broken, and in 1 minute renewed with 

 the galvanometer ; the needles deviated to the same amount 

 but with less velocity. In 3 minutes, contact being again 

 made, a deviation of 85° ensued, and in 15 minutes the ex- 

 periment was repeated and a deviation of only 4° took place. 

 Exp. 5. The cup A was connected with the positive and B 

 with the negative side of the battery during 6 minutes, and 

 then the connexion was broken. On dipping the galvanometer 

 wires into A and B, deviation of the needles to 90° ensued as 

 in the last experiment, but in the opposite direction. 



In both experiments, that platina plate which had been in 

 connexion with the negative side of the battery and had then 

 evolved hydrogen, appeared to be positive, deviating the gal- 

 vanometer needles in the same direction as a piece of zinc. 



The galvanometer made use of was extremely sensible, 

 with light and nearly astatic needles suspended by a few fibres 

 of unspun silk : the coil consisted of about 600 convolutions 

 of well annealed and insulated copper wire wound on a frame 

 on Nervander's construction. The frame being moveable on 

 its axis could be brought into any position with regard to the 

 needles. 



The temporary polar condition assumed by platina elec- 

 trodes after connexion with the battery has been before 

 pointed out by De la Rive and other physicists, a rationale 

 has also been proposed for it by Becquerel {Traite de VElec- 

 tricite, &c. 3. 109). This philosopher has only mentioned 

 the case when the electrodes have been immersed in a saline 

 solution, and after the contact with the battery is broken, con- 



