Prof. Callan on the Induction Apparatus. 329 
nary bundle of iron wires; thirdly, an elliptical or flat bundle of 
wires ; fourthly, a coil of covered iron wire; fifthly, a core con- 
sisting of a coil of insulated iron wire and of a bundle of iron 
wire ; lastly, a core consisting of two concentric coils of insulated 
iron wire, one made of fine, the other of thick wire. 
When the uninsulated iron wire coiled on a bar of iron was 
employed as a core, the spark produced by the secondary coil 
was less in length and brightness than when the iron bar alone 
was used ; because a complete circuit was formed between some 
of the spirals and those above them, whilst the other spirals were 
insulated from each other by the oxide of iron on the surface of 
the wire. 
The elliptical or flat bundle of wire receives from a given 
voltaic current flowing through a primary coil made of wire of 
given length and thickness, greater magnetic intensity than a 
cylindrical bundle does ; because when the length of the cireum- 
ference of the two bundles is the same, a section of the former 
is smaller, and contains less iron than a section of the latter. 
Therefore, if the two coils be connected with the same battery, 
the same quantity of electricity will flow through both; and the 
quantity of iron in the flat or elliptical one being less than in the 
cylindrical one, it will be more intensely magnetized. . 
I find that all cores consisting of bundles of parallel wires 
have five defects. First, in each section of every wire in such 
cores an electrical current is induced by the primary current, 
and all those currents may return to the points where they ori- 
ginated ; or there is a complete circuit for them, which is found 
to diminish the intensity of the secondary current. Some have 
imagined that by insulating the wires of the core from each 
other, they have prevented all complete circuits. But these per- 
sons seem to have forgotten, or not to have adverted to the fact, 
. that when the wires of the coil are insulated from each other, 
the primary current induces an electrical current in each section 
of every wire. 
The second defect consists in this, that the currents induced 
in each section of every wire are opposed by those in the corre- 
sponding sections of all the adjoining wires; and thus the mag- 
netic power which the primary current is capable of producing 
in the core is greatly diminished, and is less than it would be if 
all the wires were in close contact with each other; and conse- 
quently the intensity of the secondary current is diminished. 
The third defect is, that the immense quantity of electricity 
set in motion by the primary current in all the sections of each 
wire in the core is lost: it remains within the core, and cannot 
be used for producing any electrical effect. 
The fourth defect is, that we cannot ascertain the effect 
