arrangement is called a resonator or auto transformer, and is much used for producing high frequency discharges for medical purposes. The construction of a resonator is as follows: A bare copper wire is wound upon an ebonite or wooden cylinder or frame, and one end of it is connected to the outside of a Leyden jar or battery of Leyden jars, the inner coating of which is connected to one spark ball of the ordinary induction coil. The other spark ball is connected to a point on the above-named copper wire not very far from the lower end. By adjusting this contact, which is movable, the electric oscillations created in the short section of the resonator coil produce by resonance oscillations in the longer free section, and a powerful high frequency electric brush or discharge is produced at the free end of the resonator spiral. An electrode or wire connected with this free end therefore furnishes high frequency glow discharge which has been found to have valuable therapeutic powers.

The general theory of an oscillation transformer containing capacity and inductance in each circuit has been given by Oberbeck, Bjerknes and Drude.1 Suppose there are two circuits, ghegglof each consisting of a coil of wire, the two being super-T:§ ; s °" imposed or adjacent, and let each circuit contain a farmers condenser or Leyden jar in series with the circuit, and let one of these circuits contain a spark gap, the other being closed (fig. 3). If to the spark balls the secondary terminals of an ordinary induction coil are connected, and these spark balls are adjusted near one

Cz Cx another, then when the

ordinary coil is set in

M operation, sparks pass between

the balls and oscillatory

discharges take

place in the circuit containing

the spark gap.

L L These oscillations induce

I I

other oscillations in the

second circuit. The two

circuits have a certain

mutual inductance M, and

each circuit has self inductance

L1 and Lg. If

then the capacities in the

two circuits are denoted

by C, and C2 the following

simultaneous equations

express the relation of the

currents, i, and ig. and potentials, vi, and 112, in the primary and secondary circuits respectively at any instant 2-FIG. 3.

Cl, Condenser in primary circuit.

Q-1, Condenser in secondary circuit.

Inductance in primary circuit.

L2, Inductance in secondary circuit.

I-1¢E%'f'NI%'l”R1f1'l'U1 =0»

L2¢ii'l'M%'l'R2iz +112 = 0,

Rl and R2 being the resistances of the two circuits. If for the moment we neglect the resistances of the two circuits, and consider that the oscillations in each circuit follow a simple harmonic law =l sin pt we can transform the above equations into a bi quadratic LIC; +L2C2 I

4 2 ., - .

P H'c, c.<L,1., -1u2>+c, c, <L, L, -Mo'°-The capacity and inductance in each circuit can be so adjusted that their products are the same number, that is C1L, =C2L2=CL. The two circuits are then said to be in resonance or to be tuned together. In this particular and unique case the above bi quadratic reduces to

PH; 1 ik

CL ' 1 »k2

where le is written for lI~/ (L1L2) and is called the coejicient of coupling. In this case of resonant circuits it can also be shown that the maximum potential differences at the primary and secondary condenser terminals are determined by the rule V1/V2=2/C2//Cl. Hence the transformation ratio is not determined by the relative number of turns on the primary and secondary circuits, as in the case of an ordinary alternating current transformer (see TRANs1=oR1~1ERs), but by the ratio of the capacity in the two oscillation circuits. For full proofs of the above the reader is referred to the original papers. Each of the two circuits constituting the oscillation transformer taken separately has a natural time period of oscillation; that is to say, if the electric charge in it is disturbed, it oscillates to and fro in a certain constant period like a pendulum and therefore with a certain frequency. If the circuits have the same frequency when See A. Oberbeck, Wied. Ann. (1895), 55, p, 623;V. F R Bjerknes d. (1895), 55, p. 121, and (1891), 44, p. 74; and P. 'L Drude; Ann. Phys. (1904), 13, p. 512.

separated they are said to be isochronous. If n stands for the natural frequency of each circuit, where n =p/271' the above equations show that when the two circuits are coupled together, oscillations set up in one circuit create oscillations of two frequencies in the secondary circuit. A mechanical analogue to the above electrical effect can be obtained as follows: Let a string be strung loosely between two fixed points, and from it let two other strings of equal length hang down at a certain distance apart, each of them having a weight at the bottom and forming a simple pendulum. If one pendulum is set in oscillation it will gradually impart this motion to the second, but in so doing it will bring itself to rest; in like manner the second pendulum being set in oscillation gives back its motion to the first. The graphic representation, therefore, of the motion of each pendulum would be a line as in fig. 4. Such a curve

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FIG. 4.

represents the effect in music known as beats, and can easily be shown to be due to the combined effect of two simple harmonic motions or simple periodic curves of different frequency superimposed. Accordingly, the effect of inductively coupling together two electrical circuits, each having capacity and inductance, is that if oscillations are started in one circuit, oscillations of two frequencies are found in the secondary circuit, the frequencies differing from one another and differing from the natural frequency of each circuit taken alone. This matter is of importance in connexion with wireless telegraphy (see TELEGRAPH), as in ap aratus for conducting it, oscillation transformers as above described? having two circuits in resonance with one another, are employed.

REFERENCES. J. A. Fleming, The Alternate Current Transformer (2 vols., London, 1900), containing a full history of the induction coil; id., Electric Wave Telegraphy (London, 1906), dealing in chap. i., with the construction of the induction coil and various forms of interrupter as well as with the theory of oscillation transformers; A. T. Hare, The Construction of Large Induction Coils (London, 1900); ]. Trowbridge, “ On the Induction Coil, ” Phil. Mag. (1902), 3, p. 393; Lord Rayleigh, “On the Induction Coil, ” Phil. Mag. (1901), 2, p. 581; ]. E. Ives, “Contributions to the Study of the Induction Coil, " Physical Review (1902), vols. 14 and 15. (]. A. F.)

**INDULGENCE** (Lat. indulgentia, indulge re, to grant, concede),
in theology, a term defined by the official catechism of the Roman
Catholic Church in England as “the remission of the temporal
punishment which often remains due to sin after its guilt
has been forgiven.” This remission may be either total (plenary)
or partial, according to the terms of the Indulgence. Such
remission was popularly called a pardon in the middle ages a
term which still survives, e.g. in Brittany. The theory of Indulgences is based by theologians on the following
texts: 2 Samuel (Vulgate, 2 Kings) xii. 14; Matt. xvi. IQ
and xviii. 17, 18; 1 Cor. v. 4, 5; 2 Cor. ii. 6-II; but the practice itself is confessedly of later growth. As Bishop Fisher says in his Confutation of Luther, “ in the early church, faith in
Purgatory and in Indulgences was less necessary than now ....
But in our days a great part of the people would rather cast off
Christianity than submit to the rigour of the [ancient] canons:
wherefore it is a most wholesome dispensation of the, Holy
Ghost that, after so great a lapse of time, the belief in purgatory
and the practice of Indulgences have become generally received
among the orthodox ” (Confutatio, cap. xviii.; cf. Cardinal
Caietan, Tract. X V. de I ndulg. cap. i.). The nearest equivalent
in the ancient Church was the local and temporary African
practice of restoring lapsed Christians to communion at the
intercession of confessors and prospective martyrs in prison.
But such reconciliations differed from later Indulgences in at
least one essential particular, since they brought no remission of
ecclesiastical penance save in very exceptional cases. However,
as the primitive practice of public penance for sins died out in
the Church, there grew up a system of equivalent, or nominally
equivalent, private penances. just as many of the punishments
enjoined by the Roman criminal code were gradually commuted
by medieval legislators for pecuniary fines, so the years or months
of fasting enjoined by the earlier ecclesiastical codes were
commuted for proportionate fines, the recitation of a certain
number of psalms, and the like. “Historically speaking, it is

indisputable that the practice of Indulgences in the medieval