112 WRITINGS OF JOSEPH HENRY. [1838 



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 onlj'^ 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. 



17. 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 

 ribbon 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 lengthened coil begins 

 to counteract the influence of the increasing length of the 

 current. The following experiment illustrates this fact. A 

 coil of copper wire yVth 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 hun- 

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

 began to decline. This was then the proper length to pro- 

 duce 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 ribbon coil decreases. With 

 a Cruickshank's 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-flve one-inch plates, which 



