332 Prof. Callan on the Induction Apparatus. 
in view. First, to secure better insulation. The division of the 
secondary coil for the purpose of preventing the passage of 
sparks from one layer of the coil to the layer above or below it, 
was first recommended by Professor Poggendorff. Although 
this mode of preventing sparks within the coil occurred to myself 
before I saw his excellent paper on the induction apparatus, I 
was doubtful whether it would be of use, until I tried it in the 
last coil I made. My second object in dividing the secondary 
coil into parts, was to try the combined effect of the currents 
produced in each part by connecting the beginnings of all the 
parts with one coke-point, and all the ends with another. My 
third object was to try the effect of a Leyden jar connected with 
each part of the secondary wire, as well on the sparks produced 
by the part itself, as on the sparks produced by the whole 
secondary coil. 
In order to get currents of considerable quantity, and at the 
same time of very great intensity, the core and secondary coil 
should be one continuous wire, about one-eighth of an inch thick, 
and the end of the core should be connected with the beginning 
of the secondary coil. I made a flat coil of covered iron wire 
one-eighth of an inch thick. The length of the coil was about 
18 inches, its breadth 14, and its thickness between 4 and 5 inches. 
The length of the wire was about 2000 feet. On this iron coil 
I wound 150 feet of copper wire nearly one-fourth of an inch 
thick. By connecting the ends of the copper wire with a bat- 
tery of two or three 4-inch cells, and a condenser in which the 
surface of each plate was 400 square feet, sparks about the twen- 
tieth of an inch would be made to pass between the terminals of 
the iron core. [have reason to think that had the condenser been 
only one-third or one-fourth of the size, the sparks would have 
been longer. When the ends of the iron core were connected with 
a condenser in which the acting surface of each plate was about 
fifty square feet, and in which the plates were insulated from 
each other by waterproof gutta-percha cloth, the current passed 
from one plate of the condenser to the other as freely as if they 
were connected by a good conductor. When the terminals were 
connected with three large Leyden jars, the brightness of the 
spark was increased, whilst its length scarcely suffered any dimi- 
nution. I intended, but had not time, to coil over the copper 
wire another iron one of great length, and the same thickness as 
the one in the core, and to unite both together. Had I been 
able to do so, the combined currents of the core and secondary 
coil would form one of enormous intensity and considerable 
quantity. Two coils of this kind, each having a bar of iron in 
the inner iron coil, and having the ends of the iron bars con- 
nected by iron armatures, in the same way as in Mr. White- 
