260 
Prof. J. Henry’s Contributions 
tion. The electrical discharge having destroyed the insulation. 
* at some point, a part of the spires would thus form a shut 
circuit, and the induction in this would counteract the ac- 
tion in the other part of the helix ; or, in other words, the 
helix was in the same condition as the two helices mentioned 
in paragraph 70, when the ends of the wire of one were 
joined. 
76. Also the same principle appears to have an important 
bearing on the improvement of the magneto-electrical ma- 
chine; since the plates of metal which sometimes form the 
ends of the spool containing the wire, must necessarily di- 
minish the action, and also from experiment of paragraph 
72 the armature itself may circulate a closed current which 
will interfere with the intensity of the induction in the sur- 
rounding wire. I am inclined to believe that the increased 
effect observed by Sturgeon and Calland, when a bundle of 
wire is substituted for a solid piece of iron, is at least in part 
due to the interruption of these currents. I hope to resume 
this part of the subject, in connexion with several other points, 
in another communication to the Society. 
77. The results given in this section may, at first sight, 
be thought at variance with the statements of Sir H. Davy, 
that needles could be magnetized by an electrical discharge 
with conductors interposed. But from his method of per- 
forming the experiment, it is evident that the plate of metal 
was placed between a straight conductor and the needle. 
The arrangement was therefore similar to the interrupted 
circuit in the experiment with the cut plate (62.), which pro- 
duces no screening effect. Had the plate been curved into 
the form of a hollow cylinder, with the two ends in contact, 
and the needle placed within this, the effect would have been 
otherwise. 
Section V. — On the Production and Properties of induced 
Currents of the Thirds Fourth^ and Fifth Order. 
78. The fact of the perfect neutralization of the primary 
current by a secondary, in the interposed conductor, led me 
to conclude that if the latter could be drawn out, or separated 
from the influence of the former, it would itself be capable of 
producing a new induced current in a third conductor. 
79. The arrangement exhibited in fig. 8 furnishes a ready 
means of testing this. The primary current, as usual, is 
passed through coil No. 1, while coil No. 2 is placed over this 
to receive the induction with its ends joined to those of coil 
No. 3. By this disposition the secondary current passes 
through No. 3 ; and since this is at a distance, and without 
