1840] * WRITINGS OF JOSEPH HENRY. 149 



CONTKIBUTIONS TO ELECTKICITY AND MAGNETISM. No. IV. 

 ON ELECTRO-DYNAMIC INDUCTION. (CONTINUED.) 



(Transactions American Philosophical Society, n. s., vol. viii, pp. 1-35.)* 



Read June 19, 1840. 



INTRODUCTION. 



1. In the course of my last paper, (No. Ill,) it was stated 

 that the investigations which it detailed were not as complete 

 in some parts as I could wish, and that I hoped to develop 

 them more fully in another communication. After consid- 

 erable delay, occasioned by alterations in the rooms of the 

 physical department of the college, I was enabled to resume 

 m)'^ researches; and since then I have been so fortunate as to 

 discover a series of new facts belonging to different parts of 

 the general subject of my contributions. These I have an- 

 nounced to the Society at different times, as they were dis- 

 covered, and I now purpose to select from the whole such 

 portions as relate particularly to the principal subject of m}'- 

 last paper, namely, the induction at the beginning and end- 

 ing of a galvanic current, and to present them as a contin- 

 uation, and in a measure as the completion of this part of 

 my researches. The other results of my labors in this line 

 will be arranged for publication as soon as my duties will 

 permit me to give them a more careful examination. 



2. In the course of the experiments I am about to describe, 

 I have had occasion to repeat and vary those given in my last 

 paper, and I am happy to be able to state, in reference to 

 the results, that except in some minor particulars which 

 will be mentioned in the course of this paper, I have found 

 no cause to desire a change in the accounts before published. 

 My views however of the connection of the phenomena 

 have been considerably modified, and I think rendered 

 much more definite by the additional light which the new 

 facts have afforded. 



3. The principal articles of apparatus used in these ex- 

 periments are nearly the same as those described in my last 



■•^[The title-page of vol. viii bears date 1843.] 



