Page:The American Cyclopædia (1879) Volume XI.djvu/24

 16 MAGNETO-ELECTRICITY spiral placed at such a distance as to be entire- ly out of the influence of the battery current. Placing on the third a fourth (the two being separated as before by a plate of glass), and joining the ends of this with the ends of a fifth spiral, and so on, we shall have a series of successive currents. The current of the first order induced by the battery current induces a secondary oomot in the second' spiral, which passes through the third spiral, and, thus free from the influence of the battery current, in- duces a current of the third order in the fourth spiral, which in turn, passing through the fifth spiral, iii-luces a current of the fourth order in the sixth, and so on. Since each induced cur- rent must have a beginning and an ending, the current of the third order must in reality con- gist of two currents in immediate succession ami in opposite directions, one produced at the beginning and the other at the ending; and for a similar reason a current of the fourth order HUM consist of four currents in immediate suc- cession and opposite directions. On this ac- count currents of the higher orders do not definitely deflect the needle of the galvanome- ter, but merely give it a slight tremor ; the im- in opposite directions follow each other so rapidly that the inertia of the needle is not overcome in the interval between the two. The existence therefore of currents of differ- ent higher orders could not be determined by the galvanometer ; they however give intense shocks, and also permanently magnetize steel needles. This latter effect will be understood when it is recollected that, although the series of waves in different directions are the same in quantity, they differ very much in intensity ; that at the beginning of the agitation they have much the greatest energy. Hence the currents of different orders exhibit dominant impulses in definite directions. If the direc- tion of the battery current be represented by +. the current of the second order at the be- ginning of the battery current will be repre- sented by ; the dominant current of the third order +, of the fourth , and so on; while the series of dominant impulses at the ending of the battery current will be +, +, . +, , + . When a circular plate of cop- per or any other conducting substance is inter- posed between two spirals placed one above nid a current from the battery is transmitted through, for example, the lower induced current at the ending of the current "f the Lattery, in the upper spiral, will affect the galvanometer as if no plate were in- terposed, while the physiological effect, or the P wer * & n * 8hocks i wi tf be entirely neu- tralized. This remarkable effect is due to an ''nt in the interposed conductor, w Inch i-i rendered evident by cutting out a slip of the metal extending from the centre to the circumference of the plate; or in other words, hy removm- ,,m- of the radii of which the cir- cular plate may he conceived to be made up, mid thus interrupting the circuit, in which an induced current otherwise could be produced ; the shocks with the plate thus cut will be near- ly as intense as when the plate is entirely re- moved. The same effect takes place when in- stead of the plates a third flat spiral is intro- duced between the first and second spirals ; so long as the ends of this spiral are separated, its presence produces apparently no effect ; but if the ends be closed so as to form a perfect cir- cuit which can be traversed by the induced cur- rent, the power of giving shocks is neutralized. But the question naturally arises as to how the current in the plate affects the current in the upper spiral so as to destroy its power of giving shocks. The explanation of this is to be found in the fact, that while the current in the battery tends to induce a current both in the plate and in the spiral above it, each of these currents tends to induce an opposite current in the con- ductor of the other ; we may therefore consider the upper spiral as being under the -h influence of the current from the battery, and the influence of the current of the plate ; but as the current in the plate produces an equal inductive action in opposite directions at its beginning and ending, the only effect of it will be to prolong the action of the induced current in the upper spiral, or in other words, to diminish its inten- sity, and hence to neutralize its power to give shocks without perceptibly diminishing its ef- fects on the galvanometer. These facts are of importance in the construction, of the induc- tive apparatus previously described ; for if two points of two adjacent spires of the long wire happen to be in metallic contact, so as to form a closed circuit, the effect is the same as that of the interposition of a plate or spiral between the battery current and the induced current ; the intensity of the latter will be neutralized, and hence the necessity of the perfect insula- tion of the several spires of the long wire. For the same reason, if the iron core be en- closed in a hollow cylinder of copper or any other conducting metal so as to separate it from the outer coil of long wire, the great in- ductive power of the instrument will be neu- tralized ; and it is also on this account that a bundle of varnished iron wires is employed for the core instead of a solid rod of iron. If however the copper cylinder we have just mentioned be interrupted by sawing out a thin slip parallel to its axis, and the solid iron core sawed down from its circumference to its cen- tre, forming a saw-gash in the direction of the radius and in the plane of the axis, the inter- fering induced currents will be prevented. We have stated that an induced current of con- siderable intensity is generated in the conduc- tor of the battery itself at the moment of the rupture of the circuit. This also produces, on the principle of the interposed plate, an ad- verse action which tends to diminish the ener- gy of the induction apparatus ; a defect in the instrument which M. Fesso has remedied by causing the rupture to take place in a cup of mercury the surface of which is covered with