110 WRITINGS OF JOSEPH HENRY. [1838 



times formed into a ring of larger diameter, as is shown in 

 Fig. 4, Section iii. 



7. Coil No. 2 is also formed of copper plate, of the same 

 width and thickness as coil No. 1. It is however only sixty 

 feet long. Its form is shown at 6, Fig. 1. The opening at the 

 centre is sufficient to admit helix No. 1. Coils No. 3, 4, 5, 6, 

 &c., are all about sixty feet long, and of copper plate of the 

 same thickness, but of half the width of coil No. 1. 



8. Helix No 1 consists of sixteen hundred and sixty yards 

 of copper wire, ^V^h of an inch in diameter. No. 2, of nine 



Fig. 2. — a represents helix No. 1, h helix No. 2, c helix No. 3. 



hundred and ninety yards ; and No. 3, of three hundred and 

 fifty yards, of the same wire. These helices are shown in Fig. 

 2, and are so adjusted in size as to fit into each other ; thus 

 forming one long helix of three thousand yards : or, by using 

 them separately, and in different combinations, seven helices 

 of different lengths. The wire is covered with cotton thread, 

 saturated with beeswax, and between each stratum of spires 

 a coating of silk is interposed. 



9. Helix No. 4 is shown at a. Fig. 4, Section iii; it is 

 formed of five hundred and forty-six yards of wire, ^Vfh of 

 an inch in diameter, the several spires of which are insulated 

 by a coating of cement. Helix No. 5 consists of fifteen hun- 

 dred yards of silvered copper wire, xi^^h of an inch in diam- 

 eter, covered with cotton, and is of the form of No. 4. 



10. Besides these I was favored with the loan of a large 

 spool of copper wire, covered with cotton, yVf h of an inch in 

 diameter, and five miles long. It is wound on a small axis 

 of iron, and forms a solid cylinder of wire, eighteen inches 

 long, and thirteen in diameter. 



11. For determining the direction of induced currents, a 

 magnetizing spiral was generally used, which consists of 

 about thirty spires of copper wire, in the form of a cylinder, 

 and so small as just to admit a sewing needle into the axis. 



