Ampere’s experiment in figure 32 has shown us that two currents, more or less parallel, influence each other; but in 1831 Professor Faraday of the Royal Institution, London, also found that when a current is started and stopped in a wire, it induces a momentary and opposite current in a parallel wire.  Thus, if a current is started in the wire B (fig. 32) in direction of the arrow, it will induce or give rise to a momentary current in the wire A, flowing in a contrary direction to itself.  Again, if the current in B be STOPEED, a momentary current is set up in the wire A in a direction the same as that of the exciting current in B. While the current in B is quietly flowing there is no induced current in A; and it is only at the start or the stoppage of the inducing or primary current that the induced or secondary current is set up.  Here again we have the influence of the magnetic field around the wire conveying a current.

This is the principle of the “induction coil” so much employed in medical electricity, and of the “transformer” or “converter” used in electric illumination.  It consists essentially, as shown in figure 35, of two coils of wire, one enclosing the other, and both parallel or concentric.  The inner or primary coil P C is of short thick wire of low resistance, and is traversed by the inducing current of a battery B. To increase its inductive effect a core of soft iron I C occupies its middle.  The outer or secondary coil S C is of long thin wire terminating in two discharging points D1 D2.  An interrupter or hammer “key” interrupts or “makes and breaks” the circuit of the primary coil very rapidly, so as to excite a great many induced currents in the secondary coil per second, and produce energetic sparks between the terminals D1 D2.  The interrupter is actuated automatically by the magnetism of the iron core I C, for the hammer H has a soft iron head which is attracted by the core when the latter is magnetised, and being thus drawn away from the contact screw C S the circuit of the primary is broken, and the current is stopped.  The iron core then ceases to be a magnet, the hammer H springs back to the contact screw, and the current again flows in the primary circuit only to be interrupted again as before.  In this way the current in the primary coil is rapidly started and stopped many times a second, and this, as we know, induces corresponding currents in the secondary which appear as sparks at the discharging points.  The effect of the apparatus is enhanced by interpolating a “condenser” C C in the primary circuit.  A condenser is a form of Leyden jar, suitable for current electricity, and consists of layers of tinfoil separated from each other by sheets of paraffin paper, mica, or some other convenient insulator, and alternate foils are connected together.  The wires joining each set of plates are the poles of the condenser, and when these are connected in the circuit of a current the condenser