The phenomenon of electromagnetic induction consists in the appearance of an electric current in a circuit when the magnetic flux through the circuit is altered. The galvanometer needle connected to the solenoid is deflected only while the magnet is being moved into the coil. When the magnet has been moved in,
ELECTRICITY 231
all change in the magnetic field ceases and, therefore, the induction current ceases too. So in Colladon's experiment the galvanometer needle was deflected while Colladon was moving the magnet into the coil in the first room where he could not see the galvanometer. But when he left the magnet at rest and went to look at the galvanometer, the needle had had time to return to its original position.