nvestigation as to the mutual attractions and repulsions
between circuits through which electric currents are flowing, which
resulted in a theory of electro-magnetism, and finally led to the
production of the electro-magnet itself. Ampere had shown that a coil of
wire, or helix, through which an electric current is passing, acted
practically as a magnet, and Arago had magnetized an iron bar by placing
it within such a helix.

In common with the other scientific men of his time, Faraday believed
that since the flow of an electric current invariably produced
magnetism, so magnetism should, in its turn, be capable of producing
electricity. Many investigators before Faraday's time had endeavored to
solve this problem, but it was reserved to Faraday alone to be
successful. Since success in this investigation resulted from some
experiments he made while endeavoring to obtain inductive action on a
quiescent circuit from a neighboring circuit through which an electric
current was flowing, we will first briefly examine this experiment. All
his experiments in this direction were at first unsuccessful. He passed
an electric current through a circuit, which was located close to
another circuit containing a galvanometer,--a device for showing the
presence of an electric current and measuring its strength,--but failed
to obtain any result. He looked for such results only when the current
had been fully established in the active circuit. Undismayed by failure,
he reasoned that probably effects were present, but that they were too
small to be observed owing to the feeble inducing current employed. He
therefore increased the strength of the current in the active wire; but
still with no results.

Again and again he interrogates nature, but unsuccessfully. At last he
notices that there is a slight movement of the galvanometer needle at
the moment of making and breaking the circuit. Carefully repeating his
experiments in the light of this observation, he discovers the important
fact that it is only at the moment a current is increasing or decreasing
in strength--at the moment of making or breaking a circuit--that the
active circuit is capable of producing a current in a neighboring
inactive circuit by induction. This was an important discovery, and in
the light of his after-knowledge was correctly regarded as a solution of
the production of electricity from magnetism.

Observing that the galvanometer needle momentarily swings in one
dir