188 WRITINGS OF JOSEPH HENRY. [1840 



miuation of the current is more sudden, on account of the 

 prevention of the combustion of the metal by means of the 

 oil, and the fact that the end of the conductor is drawn up 

 into a non-conducting medium. 



91. The time of the subsidence of the current, when the 

 circuit is broken by means of a surface of mercury, is very 

 small, and probably does not exceed the ten-thousandth 

 part of a second, but even this is an appreciable duration, 

 since I find that the spark at the ending presents the ap- 

 pearance of a band of light of considerable length, when 

 viewed in a mirror revolving at the rat€ of six hundred 

 times in a second ; and I think the variations in the time of 

 the ending of a current under difierent conditions may be 

 detected by means of this instrument. 



92. Before concluding this communication, I should state 

 that I have made a number of attempts to verify the sug- 

 gestion given in my last paper, (No. Ill, 127.) that an inverse 

 induction is produced by a galvanic current by a change in 

 the distance of the conductors, but without success. These 

 attempts were made before I had adopted the views given 

 in this section, and since then I have found (80) a more 

 simple explanation of the alternation of the currents. 



93. In this number of my Contributions, the phenomena 

 exhibited by the galvanic apparatus have alone been dis- 

 cussed. I have however made a series of experiments on 

 the induction from ordinary electricity, and the re-action of 

 soft iron on currents; and I think that the results of these 

 can also be referred to the simple principles adopted in this 

 paper ; but they require funher examination before being 

 submitted to the public. 



