202 WRITINGS OF JOSEPH HENRY. [1842 



The same action is evidently connected with the induction 

 of a current on its own conductor, in the case of an open 

 circuit, such as that of the Ley den jar, in which the two ends 

 of the conductor are separated by the thickness of the glass. 

 And hence, if an induced current could be produced in this 

 case, one should also be obtained in that of a second con- 

 ductor, the ends of which are separated ; and this was detected 

 by attaching to the ends of the open circuit a quantity of 

 insulated metal, or by connecting one end with the earth. 



The next part of the research relates to a new examination 

 of the phenomena of the change in the direction of the 

 induced currents, with a change of distance, &c. These are 

 shown to be due to the fact that the discharge from a jar 

 does not produce a single induced current in one direction, 

 but several successive currents in opposite directions. The 

 effect on the needle is principally produced by two of these: 

 the first is the more powerful, and in the adverse direction 

 with that of the jar; the second is less powerful, and in the 

 same direction with that of the jar. To explain the change 

 of polarity, let us suppose the capacity of the needle to 

 receive magnetism to be represented by ± 10, while the 

 power of the first induced current to produce magnetism is 

 represented by — 15, and that of the second by -f 12; then 

 the needle will be magnetized to saturation or to — 10, by 

 the first induced current, and immediately afterwards all 

 this magnetism will be neutralized by the adverse second 

 induction, and a power of + 2 will remain; so that the 

 polarity of the needle in this case will indicate an induced 

 current in the same direction as that of the jar. Next, let 

 the conductors be so far separated, or the charge so much 

 diminished, that the power of the first current to develop 

 magnetism may be reduced to — 8, while that of the second 

 current is reduced to + 6, the magnetic capacity of the needle 

 remaining the same. It is evident then that the first cur- 

 rent will magnetize the needle to — 8, and that the second 

 current will immediately afterwards neutralize 6 of this, and 

 consequently the needle will retain a magnetism of — 2, or 

 will indicate an induced current in an opposite direction to 

 that of the jar. 



