In experiments like the above, it is much more convenient to wind the primary and secondary wires side by side round a bobbin, so as to form a coil. The wires are insu lated from each other by a covering of wool or silk. Not only does such a disposition admit of very long wires being used, hut it also disposes the wires employed to greater advantage, for each single turn of the primary wire acts not only on the corresponding turn of the secondary wire, but on all the turns near it. The inductive effect of such a coil is much greater than that which would be obtained by the same extent of wires run ning side by side in a straight or crooked line. It is not even necessary that the two wires be woung1 round together, each may be wound on a separate bobbin, and the one placed inside the other. The primary coil is made of wire one-twelfth of an in. in diameter, covered with wool;, and the secondary coil of silk-covered wire, about one-eightieth of an in., and much longer than. the primary Wire. two such coiis,•tiieifflustration of the preceding principles of induction can be conveniently given. If the primary coil be placed in the circuit of a galvanic cell, by two loose and flexible wires, so as to allow of its easy motion, and if the terminal binding-screws of the secondary coil be placed in connection with a galvanometer, when the primary coil is inserted into the secondary; a momentary inverse current is indicated, and when it is removed, a momentary direct one; or if, when the primary coil remains in the secondary, the strength of the primary current be altered, the needle announces the induction of currents according to the principles stated above. In order, however, to obtain the greatest effect from the sec ondary coil it is necessary, whilst the primary remains within it, to have some means of continuously completing and breaking the primary current. A contrivance for this purpose is called a rheotom, or current-break. A. simple rheotom may be made of a com mon file, by holding one wire from the battery against the end of the tile, and running the other along the teeth, the current being stopped each time the wire leaves a tooth. In this way, a rapid series of interruptions is effected, each of which is attended by an inverse and a direct current in the secondary wire. A break of the same description, but more constant, may also made. by causing a metal spring to press against the teeth of a metal wheel, both spring and wheel being connected with the battery. As the wheel is turned by a handle, the spring breaks the contact each time it slips from one tooth to another. The most convenient form of break, however, is one which is made self acting by the action of an electromagnet, which receives the name of a 'magnetic hammer.
Quantity and Tension of Induced Currents.—Let us place the primary coil within the secondary; let the primary, along with a self-acting rheotom, be put in the cir cuit of a galvanic cell, and let the secondary coil be connected with a galvanometer. The interruption in the primary current being effected by the rheotom with great rapidity, the induced inverse and direct currents are sent with corresponding rapidity through the coil of the galvanometer. If this last be of a short and thick wire, so as not to tax the tension of the current transmitted, the induced currents will not deflect the needle; or if they should happen, through the unsteady action of the break, to do so, it only oscillates round its position of rest. This proves that the quantity of elec
tricity transmitted by the induced inverse and direct currents is the same, for they each exert the same influence on the needles. But if the coil of the galvanometer consist of a long fine wire, the needle is kept deviated in a direction which argues the action of the direct current. This-leads us to conclude that both currents, though equal in quantity, are unequal in tension, the direct current having the highest tension, for it has more power to force its way through the fine wire of the galvanometer than the inverse. Other proofs of the same principles may be easily furnished.
The difference of the tension of the two induced currents is accounted for in this way: when a change takes.place in the primary current, the quantity of the electricity induced by it in the secondary wire is the sante whether this change takes place quickly or slowly; the tension, however, is very different.. When the change takes place slowly, the total quantity of electricity in circulation continues to pass as slowly, and there is little in motion at one time; but when the same occurs quickly, it is sent with momentum, so to speak, and the quantity in circulation at one time is as much greater,• in comparison with the former case, as the time is shorter. It is this quick dispatch of electricity which constitutes the tension. of the current. Now, as it takes sonic time before the primary current is fully established, the inverse induced current is slow and of low tension; 'but when the contact is broken, the primary current ceases much more suddenly than it began, and the direct induced current is quick and of high tension. This view of the matter is borne out by experiment, for it is found, that whatever favors the suddenness of the changes of the primary current, heightens the tension of the currents induced by these changes. The break, from this circumstance, forms an important element in the construction of all induction apparatus.
The inductive power of the primary coil is immensely increasedby placing a bundle of soft iron rods or wires in the center of it. The magnetism which begins and ceases in these at each passage of the current acts in conjunction with the inducing force of the coil. The center of the bobbin is hollow, to receive a bundle of this kind. The greater part of the inductive action is due to the iron core, and the induced currents got with and without it are not to be compared in point of energy. A solid bar of soft iron may also be used, but with much less advantage, for the induced currents which linger in it after the stoppage of the main current, acting themselves inductively, impair the sud denness with which the current disappears from the primary wire and magnetism from the core. The thin layer of oxide which forms on the rods insulates them sufficiently front one another, and prevents the formation of such currents. It is partly for the same reason that metal tubes cannot be used for bobbins for either primary or secondary coils. If such were used, closed circuits would be formed in them, the reaction of which would prolong the changes of the primary inducers, and consequently impair the tension of the secondary current. Metal bobbins would not be open to this objection if they had a longitudinal slit, which would make the tranverSe section a broken ring- and circuit.