334 Prof. Callan on the Induction Apparatus. 
the electricity flowing through the twenty-four thin wires. A 
third improvement may be made by winding over the primary 
coil an insulated iron or copper wire of the same length and 
thickness as the wire in the core, or of greater length, and uni- 
ting the end of the coil in the core with the beginning of the 
coil outside the primary coil. Such a coil would produce with 
a given battery a current of far greater intensity than that which 
would be produced by one of Mr. Whitehouse’s coils, or a cur- 
rent of equal intensity with a much smaller battery. It appears 
to me, then, that the use of coils such as I have described 
would be greatly to the advantage of the Atlantic Company, or 
any company having a very long telegraphic line. 
The fifth form of core which I used consisted partly of a coil 
of insulated iron wire, and partly of a bundle of iron wire. In 
one core of this form the iron wire of the coil was about the 
z4pdth of an inch, in another it was one-eighth of an inch thick. 
From the part of the core which consisted of iron wire ~}5dth of 
an inch thick, I got sparks a quarter of an inch witha single cell 
and without a condenser. The length of wire in this coil was 
about 15,000 feet. 
The sixth form of core which I used consisted of two concen- 
tric coils of insulated iron wire: one of very fine, the other of 
thick wire. The coil of thick wire should be enclosed within the 
coil of fine wire, and should be nearly 2 or 3 inches in diameter, 
especially when the primary coil is made of thick wire. In 
making coils of thick iron wire, great care is necessary, for in 
such wire there are cracks or flaws. At these cracks there are 
sometimes sharp points, which cut the covering of a spiral in an 
adjoining layer, and thus make a complete circuit, which is most 
injurious to the intensity of all the currents induced in the 
various parts of the coil. It is necessary to know that the com- 
plete circuit which diminishes the intensity of the secondary 
current in the greatest degree, is that which is made by connect- 
ing the ends of a coil of thick wire. I have not had time to 
determine which of the forms of core I have used induces the 
most intense current in the secondary coil, or which of them 
makes the condenser act with the greatest effect. I once used 
for the core a bundle of wires, 9 inches in diameter and 26 
inches long. The weight of the core exceeded two hundred and 
ahalf pounds. Thiscore acted so badly, as to convince me that 
anyone who wishes to obtain currents of very great intensity, 
or very long sparks, should never employ cores of very large 
diameter. 
The fourth result is an improved method of insulation for the 
secondary coil. In this mode the insulation is imperfect where 
imperfect insulation is sufficient, and perfect where such insu- 
