ON ELECTRO-DYNAMIC INDUCTION. 305 



nated by the names of coil No. 1, coil No. 2, &c. ;'also of several coils 

 of long wire ; and these, to distinguish them from the ribands, will be 

 called helix No. 1, helix No. 2, &c. 



6. Coil No. 1 is formed of thirteen pounds of copper plate, one inch 

 and a half wide and ninety-three feet long. It is well covered with 

 two coatings of silk, and was generally used in the form represented in 

 Fig. 1, which is that of a flat spiral sixteen inches in diameter. It 

 was however sometimes 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 cop- 

 per wire, 4Vth of an inch in diameter. No. 2, of nine hundred and 



Fig. 2. ninety yards ; and No. 3, of three 



hundred and fifty yards, of the same 

 wire. These helices are shown in 

 -^-^ \ J o Fig. 2, and are so adjusted in size 



a represents helix No. 1, h helix No. 2, c helix No. 3. aS tO fit into Cach othcr ; thuS form- 

 ing one long helix of three thousand yards : or, by using them sepa- 

 rately, 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. HelixNo. 4 is shown at «, Fig. 4, Section III. ; it is formed of five hun- 

 dred and forty-six yards of wire, 4Vth of an inch in diameter, the several 

 spires of which are insulated by a coating of cement. Helix No. 5 

 consists of fifteen hundred yards of silvered copper wire, TrWh of an inch 

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



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

 copper wire, covered with cotton, ^\-th of an inch in diameter, and five 

 miles long. It is wound on a small axis of iron, and forms a solid cy- 

 linder of wire, eighteen inches long, and thirteen in diameter. 



11. For determining the direction of induced currents, a magnetiz- 



