Prof. Callan on the Induction Apparatus. 327 
broken. Now in every magnet, at the moment of the cessation 
of its magnetism, an electric current is produced in a direction 
at right angles to the magnetic axis, in the magnet itself and in 
all contiguous bodies. First, it has been already shown that at 
the moment iron loses its magnetic power, an electric current is 
produced in each section of it in a direction perpendicular to its 
magnetic axis. By the laws of induction, these currents induce 
parallel ones in every contiguous conductor. Secondly, when a 
current flowing from a battery through a copper wire ceases, the 
wire loses its magnetism ; and it is found by experiment, that. at 
the moment of losing its magnetism, an opposite electrical current 
is produced in the whole length of the wire, or in a direction at 
right angles to its magnetic axis. Hence, because in every in- 
duction coil excited by a battery there are three magnets, viz. 
the core, the primary and secondary coils, having a common axis, 
and because at the moment the connexion with the battery is 
broken the three lose their magnetic power, an electrical current 
is produced in each section of each of the magnets in a direction 
perpendicular to their common axis; and these currents in each 
magnet induce electrical currents in the other two. Therefore, 
when the connexion with the battery is broken, a current is pro- 
duced in the secondary coil, which is the result of the combined 
action of three inductive forces arising from the suspension of 
the magnetism of the core, of the primary and of the secondary 
coil. When the secondary coil is made of iron wire, the mag- 
netic power it will receive from the primary current, and from 
the magnetic inductive force of the core, will be far greater than 
if it be made of copper wire; and therefore the intensity of the 
secondary current in a coil of iron wire must be much greater 
than that of the secondary current in a coil of copper wire. I 
showed, at the late meeting of the British Association in Dublin, 
an induction coil in which the secondary wire was of iron: its 
length was about 21,000 feet, and its thickness about the ;>5dth 
of an inch. With a single cell, 6 inches by 4, and without a 
condenser, this coil gave sparks half an inch long. Shoulda ~ 
condenser of the proper size increase the length of the sparks, as 
it does in Mr. Gassiot’s great coil, in a thirtyfold ratio, my coil 
ought to give sparks 15 inches long with a single cell. I have 
not yet tried it with a condenser: I made two large condensers, 
in which, when both were united, the acting surface of each plate 
exceeded 600 square feet. After being used for some time, the 
insulation of the plates gave way, and the action of the con- 
denser became feeble, and once ceased altogether. I intend to 
reconstruct both condensers as soon as possible, and to try their 
effect on the coil, on which I have, since the meeting of the 
Association, coiled about 28,000 feet more of fine iron wire, so 
