Prof. Callan on the Induction Apparatus. 335 
lation is required, and consequently each spiral is brought 
nearer to the core, to the primary coil, and to the other spirals 
of the secondary coil, than in the ordinary manner of in- 
sulation, in which the parts of each layer for which very little 
insulation is required are as well insulated from the layer above 
and below it as the parts which require the best insulation. 
My mode of insulation differs from the ordinary one in two 
respects :—First, in the insulation of each spiral from the adjoin- 
ing ones in the same layer; secondly, in the insulation of the 
spirals of every layer from the contiguous spirals of the layer above 
it. I do not cover the fine wire with thread of any kind; but I 
coat it with a very thin film of varnish by drawing it through 
melted rosin and bees-wax. I draw it through the hot varnish 
by winding it on the coil at the distance of about 25 feet from 
the stove by which the varnish is heated; I have found that at 
this distance the varnish is cool and hard, even when the wire is 
drawn through it at the rate of 8000 feet in the hour. Thus in 
this mode of insulating the fine wire, a coil may be made in a 
comparatively short time. The insulation is sufficient, because 
the difference between the intensity of any spiral and the ad- 
joining ones of the same layer is indefinitely small. On every 
inch of each layer I can put eighty or eighty-two spirals of 
wire ;4,5dth of an inch thick. My mode of insulating the spi- 
rals of each layer from those of the layer above or below it, 
differs also from the way in which they are insulated by others. 
In the common mode of insulation, if, as in Mr. Gassiot’s 
great coil, five thicknesses of gutta-percha, or of any other in- 
sulating substance, be thought necessary in order to insulate 
the extreme spirals of any layer from those of the layer below 
it, five thicknesses of the insulating substance are put between 
the whole length of every two adjoining layers; so that if there 
be twenty layers along with the first, there will be 100 thick- 
nesses of the insulating substance. But, in my mode of insula- 
tion, there would, in such a case, be only sixty. In order 
to render my mode of insulation intelligible, I shall explain how 
the first layer of spirals is insulated from the second, and the 
second from the third. Every other layer, such as the third, 
fifth, seventh, &c. represented by an odd number, will be insu- 
lated from the one above it, in the same way as the first is insu- 
lated from the second ; and every layer, such as the fourth, sixth, 
eighth, &c. represented by an even number, will be insulated 
from the one above it, in the same way as the second is from the 
third. In insulating the first layer from the second, when five 
thicknesses of the insulating substance to be used are deemed 
necessary for the insulation of the last spirals of the second layer 
from the first spirals of the first (there the difference of intensity 
