204< Prof. J. Henry's Contrihitions 



more definitely the conditions which influence the action of 

 the spiral conductor. 



13. When tlie electricity is of low intensity, as in the case 

 of the thermo-electrical pile, or a large single battery weakly 

 excited with dilute acid, the flat riband coil No. 1, ninety- 

 three feet long, is found to give the most brilliant deflagra- 

 tions, and the loudest snaps from a surface of mercury. The 

 shocks, with this arrangement, are, however, very feeble, and 

 can only be felt in the fingers or through the tongue. 



16. The induced current in a short coil, which thus pro- 

 duces deflagration, but not shocks, may, for distinction, be 

 called one of quantity. 



1 7. When the length of the coil is increased, the battery 

 continuing the same, the deflagrating power decreases, while 

 the intensity of the shock continually increases. With five 

 riband coils, making an aggregate length of three hundred 

 feet, and the small battery, fig, 1, the deflagration is less than 

 with coil No. 1, but the shocks are more intense. 



18. There is, however, a limit to this increase of intensity 

 of the shock, and this takes place when the increased resist- 

 ance or diminished conduction of the lengthenedj coil begins 

 to counteract the influence of the increasing length of the 

 current. The following experiment illustrates this fact. A 

 coil of copper wire y'gth of an inch in diameter, was increased 

 in length by successive additions of about thirty-two feet at 

 a time. After the first two lengths, or sixty-four feet, the 

 brilliancy of the spark began to decline, but the shocks con- 

 stantly increased in intensity, until a length of five hundred 

 and seventy-five feet was obtained, when the shocks also be- 

 gan to decline. This was then the proper length to produce 

 the maximum effect with a single battery, and a wire of the 

 above diameter. 



19. When the intensity of the electricity of the battery is 

 increased, the action of the short riband coil decreases. With 

 a Cruickshanks trough of sixty plates, four inches square, 

 scarcely any peculiar effect can be observed, when the coil 

 forms a part of the circuit. If however the length of the 

 coil be increased in proportion to the intensity of the current, 

 then the inductive influence becomes apparent. When the 

 current, from ten plates of the above-mentioned trough, was 

 passed through the wire of the large spool (10.), the induced 

 shock was too severe to be taken through the body. Again, 

 when a small trough of twenty-five one-inch plates, which 

 alone would give but a very feeble shock, was used with helix 

 No. 1, an intense shock was received from the induction 

 when the contact was broken. Also a slight shock in this 



