disorders. When sent through glass tubes filled with rarefied
gases, sometimes called "Geissler tubes," they elicit glows of
many colours, vieing in beauty with the fleeting tints of the
aurora polaris, which, indeed, is probably a similar effect of
electrical discharges in the atmosphere.

The action of the induction is reversible. We can not only send a
current of low "pressure" from a generator of weak electromotive
force through the primary coil, and thus excite a current of high
pressure in the secondary coil, but we can send a current of high
pressure through the secondary coil and provoke a current of low
pressure in the primary coil The transformer or converter, a
modified induction coil used in distributing electricity to
electric lamps and motors, can not only transform a low pressure
current into a high, but a high pressure current into a low. As
the high pressure currents are best able to overcome the
resistance of the wire convening them, it is customary to transmit
high pressure currents from the generator to the distant place
where they are wanted by means of small wires, and there transform
them into currents of the pressure required to light the lamps or
drive the motors.

We come now to another consequence of Oersted's great discovery,
which is doubtless the most important of all, namely, the
generation of electricity from magnetism, or, as it is usually
called, magneto-electric induction. In the year 1831 the
illustrious Michael Faraday further succeeded in demonstrating
that when a magnet M is thrust into a hollow coil of wire C, as
shown in figure 37, a current of electricity is set up in the coil
whilst the motion lasts. When the magnet is withdrawn again