Electro-physiology and electro-therapeutics : showing the best methods for the medical uses of electricity / By Alfred C. Garratt.
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It has already been shown that when contact is made with the battery, the current which.passes through the inner primary coarse wire determines in the second or outer longer and finer wire an instantaneous current at the moment when it begins to circulate, and another equally instantaneous current at the mo- ment when it ceases to pass. Now, here is the peculiarity of this current from electric induction; i. c, it is thus always instanta- neous, and that at the instant the inducing current starts, and again when it stops. The same is true of the currents of mag- netic induction. But, fortunately, the magnetic current can act not only instantaneously and simultaneously, but also in the same direction with the other bits of currents ; for these latter are not produced whilst the soft iron remains a magnet, but rather only when it gains and loses its magnetism. The demag- netization of the soft iron within the helix machine, therefore, must have the same effect as breaking the current of the battery in the production of an equally instantaneous current. It is clearly understood, then, that the current of the battery which circulates in the coarse wire of the helix never arrives at the patient. It is only the induced currents (from the two sources, namely, from the within magnetized soft iron, and the without long fine coil of the helix that is rendered electric simultaneously by the mutual action of the battery and the magnet) that are felt at the electrodes. The galvanometer indicates exactly, not only the existence of such currents as being only and always instan- taneous, but also their direction; for if we compare the direction of the different currents, we find that the induced current — i.e., the one in the fine long wire that leads to the electrodes on making the circuit — is contrary to that of the current of the battery ; while the direction of the one on breaking the circuit is in the same direction with that of the battery. We therefore find the law, that these kinds of currents are but shocks, of only instantaneous duration, and that the direction of each alternate shock or current is invariably and necessarily in the contrary direction. To render such an arrangement available as a kind of cur- rent, there must be some contrivance for breaking and making