2 placed two secondary coils, S1, S2, of wire, of the same size, and of exactly equal numbers of turns of wire. The secondary coils are joined to a receiver T, and are wound in opposite directions. The result of this arrangement is that whenever a current either begins or stops flowing in the primary coils, P, induces a current in S1, and P2 in S. As S and S, are wound in opposite ways, the two currents thus induced in the secondary wire neutralize one another, and, if they are of equal strength, balance one another so exactly that no sound is heard in the telephone. But a perfect balance cannot be obtained unless the resistances and the coefficients of mutual induction and of self-induction are alike. If a flat piece of silver or copper (such as a coin) be introduced between S, and P1, there will be less induction in S, than in S2, for part of the inductive action in P, is now spent on setting up currents in the mass of the metal (Art. 459), and a sound will again be heard in the telephone. But balance can be restored by moving S, farther away from P2, until the induction in S, is reduced to equality with S1, when the sounds in the telephone again cease. It is possible by this means to test the relative conductivity of different metals which are introduced into the coils. It is even possible to detect a counterfeit coin by the indication thus afforded of its conductivity. The induction balance has also been applied in surgery by Graham Bell to detect the presence of a bullet in a wound, for a lump of metal may disturb the induction when some inches distant from the coils. CHAPTER XIV ELECTRIC WAVES LESSON LIV. - Oscillations and Waves 515. Electric Oscillations. If a charged condenser or Leyden jar is discharged slowly through a conductor of high resistance, such as a nearly dry linen thread, the charge simply dies away by a discharge which increases in strength at first, and then gradually dies away. If, however, the condenser is discharged through a coil of wire of one or more turns (the spark being taken between polished knobs to prevent premature partial discharges by winds or brushes) the effect is wholly different, for then the discharge consists of a number of excessively rapid oscillations or surgings. This is in consequence of the self-induction of the circuit, by reason of which (Art. 458) the current once set up tends to go on. The first Fig. 290. rush more than empties the condenser, and charges it the opposite way; then follows a reverse discharge, which also overdoes the discharge, and charges the condenser the same way as at first, and so forth. Each successive oscil lation is feebler than the preceding, so that after a number of oscillations the discharge dies away as in Fig. 290. The spark of a jar so discharged really consists of a number of successive sparks in reverse directions. One proof of this, as pointed out by Henry in 1842 from the experiments of Savery, is that if jar discharges through a coil are used to magnetize steel needles, the direction of the magnetization is anomalous, being sometimes one way, sometimes the other. That a discharge ought under certain conditions to become oscillatory was noted by von Helmholtz. Lord Kelvin in 1855 predicted these conditions. If the capacity of the condenser is K (farads), the resistance of the circuit R (ohms), and its inductance L (henries), there will be oscillations if R<√4L/K; and there will be no oscillations if R>√4L/K. In the former case the frequency n of the oscillations will be such that Example.-If K = 0.01 microfarad, L = 0·00001 henry, and If R is small n is nearly equal to 1 + 2π VKL. The oscillations can be made slower by increasing either K or L. The oscillations of an ordinary Leyden jar discharge may last only from a ten-thousandth to a tenmillionth of a second. By using coils of well-insulated wire and large condensers, Lodge has succeeded in slowing down the oscillations to 400 a second; the spark then emitting a musical note. Iron is found to retain its magnetic properties even for oscillations of the frequency of one million per second. Feddersen subsequently examined the spark of a Leyden jar by means of a rotating mirror, and found that instead of being a single instantaneous discharge, it exhibited definite fluctuations.* With very small resistances in the circuit, there was a true oscillation of the electricity backward and forward for a brief time. The period of the oscillations was found to be proportional to the square root of the capacity of the condenser. With a certain higher resistance the discharge became continuous but not instantaneous. With a still higher resistance the discharge consisted of a series of partial intermittent discharges, following one another in the same direction. Such sparks when viewed in the rotating mirror showed a series of separate images at nearly equal distances apart. 516. Electric Waves. Though the increasing and dying away of currents, for example in cables, is sometimes loosely described as of "waves" of current, these phenomena are very different from those of true electric or electromagnetic waves propagated across space. In the case of true electric waves, portions of the energy of the current or discharge are thrown off from the conductor and do not return back to it, but go travelling on in space. If a current increases in strength the magnetic field around it also increases, the magnetic lines enlarging from the conductor outward, like the ripples on a pond. But as the current is decreased the magnetic lines all return back and close up upon the conductor; the energy of the magnetic field returns back into the system. But if for currents slowly waxing and waning we substitute electric oscillations of excessive rapidity, part of their energy radiates off into the surrounding medium as electromagnetic waves, and only part returns back. As will be presently set forth, these waves possess all the optical properties of light-waves, and can be reflected, refracted, polarized, etc. It is a fundamental part of the modern views of electric action that while an electric displacement (Art. 57) is These electric oscillations were examined also by Schiller, Overbeck, Blaserna, and others, notably by Hertz; see Art. 520 below. being produced in a dielectric, the effect in surrounding space is the same as if there had been a conductive instead of an inductive transfer of electricity. Maxwell gave the name of displacement-current to the rate of change of the displacement. Experiment proves that displacement-currents, while they last, set up magnetic fields around them; just as convexion-currents (Art. 397) and conduction-currents do. 517. Resonance. The circumstance that when certain definite relations exist between the capacity and inductance of a circuit and the frequency of the periodic currents, the choking reactions of these properties neutralize one another, has been already alluded to in Art. 473. And we have seen (Art. 515) that a circuit with a certain self-induction, capacity, and resistance tends to oscillate electrically at a certain frequency. If it be placed in a medium through which electric waves of that frequency are passing in such a position that the electric and electromagnetic fields of the successive waves can induce currents in it, each wave will give a slight impulse to the readily-excited oscillations, which will grow in intensity, just as small impulses given to a pendulum at the right times will make it swing violently. S The following experiment of Oliver Lodge beautifully illustrates this phenomena of resonance, and at the same time the production of waves by an oscillatory discharge. Two Leyden jars, Fig. 291, are placed a little way apart from one another. One of them, charged from an influence machine not shown, is provided with a bent wire to serve as a discharging circuit, with a spark-gap S A R Fig. 291. The second jar is between the polished knobs at the top. provided with a circuit of wire, the inductance of which |