Prof. Callan on the Induction Apparatus. 337 
each of these secondary coils there will be nearly 60,000 feet of 
iron wire about the ;3,5dth of an inch thick. 
The fifth result is a contact-breaker in which the striking 
parts are copper, and which act as well as if they were platina. 
The contact breaker consists, first, of a small electro-magnet ; 
secondly, of its armature screwed toa board moveable on a hinge, 
and having attached to it a spring connected with the vibrating 
piece of copper; thirdly, of a spring for pressing the striking 
pieces together; and of a trough containing oil, in which 
these pieces are always immersed. By means of the spring 
attached to the board to which the armature is fastened, the ar- 
mature is brought within the most convenient distance from the 
small electro-magnet. The spring presses the striking pieces 
together with the greatest force the electro-magnet is capable of 
overcoming, and the pressure is exerted immediately over the 
points of contact. The oil prevents in some measure the oxidation 
of the copper, and serves to stop the battery current more quickly ; 
for as soon as the pieces of copper are separated, the oil rushes 
in between them, and being a non-conductor, instantly stops the 
galvanic current from the battery. In the first contact-breaker 
which I made of this kind, there were two vibrating pieces, one 
of platina, the other of copper; the former struck against 
another piece of platina, the latter against a piece of copper: 
the copper was immersed in oil. By means of two screws, 
both might be made to make and break contact together, or 
I could cause either to make and break contact. By first 
causing the platina, and afterwards the copper, to make and 
break contact, I found that the copper acted as well as the 
platina. In the contact-breaker which I showed at the meeting 
in Dublin, there were three vibrating pieces of copper, each about 
three-eighths of an inch thick. M. Foucault thinks that the 
contact. will be made and broken as well by one as by several 
vibrating pieces. Though that should be the case, the addition 
of two other pieces will not be useless; for the three may be 
immersed in different fluids, and thus we can discover the fluid 
in which contact may be made and broken with the greatest 
advantage. 
The sixth result is a more satisfactory explanation of the con- 
denser. In order to understand the action of the condenser, we 
must examine the electrical state of the primary coil at the mo- 
ment its connexion with the battery is broken, and the effect 
which this state has on the core and secondary current. At the 
moment the connexion. between the battery and primary coil is 
broken, the electricity which it received from the battery con- 
tinues to flow to the end of the coil to which it was moving ; but 
being no longer urged forward by the battery, its velocity 
Phil. Mag. 8. 4. Vol. 14. No. 94, Nov, 1857. Z 
