Prof. Henry's Contribtdions to Electricity and Magnetism. 483 



cups. A zinc rod is suspended in each of these, of the same 

 length as the cylinders, and about one inch in diameter. The 

 several elements are connected by a thick copper wire, sol- 

 dered to the copper cylinder of one element, and dipping 

 into a cup of mercury on the zinc of the next. The copper 

 and zinc, as usual, are separated by a membrane, on bodi sides 

 of which is placed a solution of one part of sulphuric acid in 

 ten parts of water ; and to this is added, on the side next the 

 copper, as much sulphate of copper as will saturate the solu- 

 tion. The battery was sometimes used as a single series, with 

 all its elements placed consecutively, and at others in two or 

 three series, arranged collaterally, so as to vary the quantity 

 and intensity of the electricity as the occasion might require. 



4. The galvanometers mentioned in this paper, and re- 

 ferred to in the last, are of two kinds ; one, which is used 

 with a helix, to indicate the action of an induced current of 

 intensity, consists of about five hundred turns of fine copper 

 wire, covered with cotton thread, and moi'e effectually insu- 

 lated by steeping the instrument in melted cement, which was 

 drawn into the spaces between the spires by capillary attrac- 

 tion. 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 in- 

 tensity. 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 



ieeble intensity. 



Section I. — On the Induction produced at the viomcnt of the 



beginning of a Galvanic Current, 8^c. 



6. It will be recollected that the arrangement of apparatus 



employed in my last series of experiments gave a jjowerful 



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 in- 



2 I 2 



