108 Mr. Joule on the Intermittent Character of the Voltaic 



not deceived, for on examining the amalgamated zinc I ob- 

 served the following very curious phenomenon : — At intervals 

 of one or two seconds a white shade overspread the surface of 

 the amalgamated zinc and then suddenly disappeared, leaving 

 the metal brilliant. The pulsations of the current were evi- 

 dently simultaneous with these sudden changes in the appear- 

 ance of the electrode, and the needle received a sudden im- 

 pulse every time the white film suddenly broke away. 



All the above experiments were made more than a year ago, 

 those which follow are the experiments I have recently made 

 in the investigation of the same subject. 



A plate of amalgamated zinc and a stout iron wire were 

 immeysed in a solution consisting of one part strong oil of 

 vitriol to six parts of water. The iron was connected with the 

 positive, the zinc with the negative electrode of a battery of 

 five large Daniell's cells, and a galvanometer was included in 

 the circuit. The instant that the circuit was completed a 

 powerful current was transmitted through it, hydrogen being 

 evolved from the negative zinc, whilst the positive iron was oxi- 

 dized and began to dissolve away. In a short time, however, 

 the intensity of the current began to decline very rapidly, and 

 the iron electrode, ceasing to be dissolved, assumed the pass- 

 ive state* described by Schcenbein, and began to evolve 

 oxygen gas, and continued to do so as long as I had patience 

 to watch it. On breaking the circuit and then closing it 

 afresh, the same phaenomena were repeated. Having now re- 

 duced the battery from five to three cells, the action of the iron 

 became intermittent. First it was dissolved, the needle being 

 at the same time deflected 45° ; then it began to evolve oxygen, 

 the needle at the same time declining rapidly until it stood at 

 15°; and then again the oxygen suddenly ceased to be evolved, 

 while at the same moment the needle sprang forwards and 

 began to oscillate about its former resting place at 45°. The 

 iron remained in each state about half a minute, and a white 

 film was observed to pass over its surface every time that the 

 oxygen was about to rise, and to disappear suddenly when the 

 evolution of oxygen ceased. 



Having watched these curious phaenomena for some time, 

 it occurred to me to try the effect of connecting two cells with 

 the battery so as to divide the current between two positive 

 iron electrodes. On making the experiment I found that the 

 action of the iron was in both cells intermittent ; and, what 

 was very remarkable, that the condition of the iron electrodes 



* Keir appears to have been the first to observe the passive state of iron. 

 Phil. Trans, for 1790. [Keir's priority of observation has already been 

 pointed out by Sir John Herschel and Mr. Faraday : see Phil. Mag. S. 3. 

 vol. ix. p. 122, vol. xi. p. 333.— Edit.] 



