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by introducing into the interior of the hollow helix a magnet
instead of the electro-dynamic coil; and this he demonstrated
to be so, and that even in a much stronger degree. In fact,
the electric current that traverses one of the helices or coils of
the machine at the moment when it is established, not only de-
Fig. 1G. An Electro-Magnetic Double Helix, with Clock-work for interrupting the Primary or
Battery Current.
(ermines a current of electro-dynamic induction in the other
coil of the helix, but at the same time, if there is placed a soft
iron within the helix, it magnetizes it; and for this same reason
also determines in the second coil a magneto-electric current in
the same direction, and much more powerful. So, when the cur-
rent ceases to pass, the soft iron being instantly demagnetized,
there is developed a second induced current from the magnet,
which is now also added to that which results from the direct
effect on the second wire in the helix — all proceeding from
the inducing current that is flowing from the battery through
the first wire in the helix.
The first physician who employed induction currents for medi-
cal purposes was Dr. Neef, of Frankfort-on-the-Main. He first
invented the automatic vibrating spring for interrupting the
current, which indeed opened a new era in the therapeutical uses
of electricity.
