150 WRITINGS OF JOSEPH HENRY. * [1840 



paper, namely, several flat coils, and a number of long wire 

 helices. (No. Ill, 6, 7, 8.*) I have however added to these a 

 constant battery, on Prof. Daniell's plan, the performance of 

 which has fully answered my expectations, and confirmed 

 the accounts given of this form of the instrument by its 

 author. It consists of thirty elements, formed of as many 

 copper cylinders, open at the bottom, each five inches and a 

 half in height, three inches and a half in diameter, and 

 placed in earthen 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, soldered to the copper 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 both 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 solution. The battery was sometimes used as a 

 single series, with all its elements placed consecutively, and 

 at others in two or three series, arranged collaterall}', 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 in- 

 tensity, consists of about five hundred turns of fine copper 

 wire, covered with cotton thread, and more efFectuall}'' insu- 

 lated 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 quan- 

 tity, but of feeble intensity. The needle of both these in- 

 struments is suspended by a single fibre of raw silk. 



5. I should also state, that in all cases where a magnetiz- 

 ing spiral is mentioned in connection with a helix, the 



*The numerals II or III included in parentheses refer to the correspond- 

 ing Nos. of my previous Contributions. 



