336 Prof. Callan on the Induction Apparatus. 
is greatest), I divide the length of the layer into five equal parts. 
I then put one thickness of the insulating substance (let us sup- 
pose it to be what I use, viz. the paper employed for copper-plate 
engravings saturated with a solution of gutta-percha im oil) on 
the entire length of the first layer, and then roll the fine wire on 
one fifth of the layer. I next cover the whole length of the coil 
with another thickness of prepared paper, and coil the fine wire 
on the second fifth of the layer. I then put on a third thick- 
ness of paper, and wind the wire on the third fifth of the coil. 
I then put on another thickness of paper, and coil the wire on 
the fourth fifth, and so on. Then between the first fifth of the 
second layer and the spirals below it in the first, there is one 
thickness of paper; and one will insulate them as well as five 
will insulate the whole length of the two layers from each other. 
Between the second fifth of the second layer and the part of the 
first layer below it, there are two thicknesses of paper, and they 
will sufficiently insulate these two parts from each other, In 
the same way the third is insulated by three thicknesses, the 
fourth by four, and the last by five thicknesses of paper: thus 
the five parts of the coil are as well insulated from each other as 
if there were five thicknesses between the entire length of the 
two layers. To insulate the second layer from the third, as well 
as the first is insulated from the second, only one thickness of 
paper is necessary ; for by putting a single thickness of paper 
on the second layer, the first fifth is covered by one, the second 
by two, the third by three, the fourth by four, and the last by 
five thicknesses of paper. Hence to insulate any two layers, 
only six thicknesses of the insulating substance are necessary, 
or three for the insulation of each layer; and therefore to insu- 
late twenty layers, only sixty thicknesses of the insulating sub- 
stance to be used are required. Thus in my mode of insulation, 
every spiral in the secondary coil is brought nearer to all the 
contiguous spirals and to the primary coil and core, than in 
the ordinary method of insulation; and consequently the in- 
ductive power of the core and of the primary coil on the 
secondary one, as well as the inductive power of the spirals of the 
secondary coil on each other, must produce a secondary current 
of far greater intensity in mine than in the common mode of 
insulation. The coil which was shown at the meeting of the 
British Association was insulated in the manner just explained. 
This coil and the contact-breaker, which will be presently de- 
scribed, were seen at work by Mr. Gassiot, Dr. Robinson, 
M. Foucault, Professor Rogers, and other members of the Asso- 
ciation. Mr. Gassiot was so much pleased with their action and 
construction, that he ordered from Mr. Yeates, an optician in 
Dublin, a contact-breaker and two secondary coils like mine, In 
