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as the ends of the arms of the little magnet, for either grease
or rust will stop its action. The contacts must all be carefully
attended to. Much power is often lost by this neglect, besides
the inconvenience of uncertain action, or no action at all.
Go where we will, as I have said, we find a great variety
of these electro-magnetic machines. Nevertheless, they pos-
Fig. 20. Electro-Magnetic Portable Machine, complete in its Box.
sess in the main generally the same principle, and about the
same internal arrangement or property for obtaining in a more
or less degree an intermitting and alternating current of inten-
sity, simply from a single galvanic battery of great relative
quantity.
The intensity of these induction currents from a given
machine depends, in the first place, upon the intensity of the
battery action. If this is any way feeble, it will not develop a
powerful magnetism ; therefore the extra current from this
source, as well as the current induced in the second or outer
wire will be of low tension. The current can be made more
