ON ELECTRO-DYNAMIC INDUCTION. 3 



induced current of intensity, consists of about five hundred turns of fine copper 

 wire, covered with cotton thread, and more effectually insulated by steeping the 

 instrument in melted cement, which was drawn into the spaces between the 

 spires by capillary attraction. The other galvanometer is formed of about 

 forty turns of a shorter and thicker wire, and is always used to indicate an 

 induced current, of considerable quantity, but of feeble intensity. The needle 

 of both these instruments is suspended by a single fibre of raw silk. 



5. I should also state, that in all cases where a magnetizing spiral is mentioned 

 in connexion with a helix, the article is formed of a long, fine wire, making 

 about one hundred turns around the axis of a hollow piece of straw, of about 

 two inches and a half long : also the spiral mentioned in connexion with a coil, 

 is formed of a short wire, which makes about twenty turns around a similar 

 piece of straw. The reason of the use of the two instruments in these two 

 cases is the same as that for the galvanometers, under similar circumstances, 

 namely, the helix gives a current of intensity, but of small quantity, while the 

 coil produces one of considerable quantity, but of feeble intensity. 



Section I. 

 On the Induction produced at the moment of the Begi?ming of a Galvanic Current, dfC. 



6. It will be recollected that the arrangement of apparatus employed in my 

 last series of experiments gave a powerful induction at the moment of breaking 

 the galvanic circuit, but the effect at making the same was so feeble as scarcely 

 to be perceptible. I was unable in any case to get indications of currents of 

 the third or fourth orders from the beginning induction, and its action was 

 therefore supposed to be so feeble as not materially to affect the results ob- 

 tained. 



7. Subsequent reflection, however, led me to conclude, that in order to com- 

 plete this part of my investigations, a more careful study of the induction at 

 the beginning of the current would be desirable, and accordingly, on resuming 

 the experiments, my attention was first directed to the discovery of some means 

 by which the intensity of this induction might be increased. After some pre- 

 liminary experiments, it appeared probable that the desired result could be ob- 

 tained by using a compound galvanic battery, instead of the single one before 

 employed. In reference to this conjecture the constant battery before men- 

 tioned (3) was constructed, and a series of experiments instituted with it, the 

 results of which agreed with my anticipation. 



