time is necessary both to charge and to discharge it. This confusion is made very evident if we attempt to use a telephone and speak over the cable. At a distance of about fifty miles nothing but a murmur is heard; the characteristics of the human voice are completely obliterated by the capacity of the cable. In order to speak under the ocean, some way must be found of neutralizing this capacity. To the electrician whose work lies in the field of telephony the subject of the Leyden jar therefore is of the utmost importance. Meanwhile, to obviate the disturbing effect of the capacity of the cable we do not connect our battery direct to the cable, but, having first charged a condenser by connecting its two coatings to a battery, we discharge the condenser into the cable and use very delicate instruments to record the feeble current which thus traverses the cable. The process of submarine telegraphy is thus simply one of discharging a Leyden jar through it. A comparatively large electro-motive force can thus be used without the danger of heating the wire of the cable and without giving the cable too great a charge.

In the submarine cable, therefore, the practical electrician sees the importance of the study of the Leyden jar. To the scientific man its manifestations have become of greater importance than the electrical effects we have hitherto studied in metallic circuits, for he is persuaded that by the study of the electrical phenomena in the dielectric-the glass, for instance, of a Leyden jar—we shall more clearly understand the relationship between light, heat, and electricity. In an ordinary voltaic cell we can trace the complete circuit of electrical phenomena. The electro-motive force arises in some mysterious way from the two metals of the bat

tery; a current results from this force and the chemical actions in the cell. This current can be traced on the wire connecting the two metals of the cell and in the liquid of the cell. There is no part of this circuit where there is no evidence of an electric current. It is present in the outer wire which rings our house bell; it is evident in the liquid of the cell. In the case, however, of a Leyden jar the current manifests itself apparently only in the wire connecting the outer and inner coating of the jar. The glass that separates these coatings is called an insulator; it does not permit a current of electricity to pass through it. How, then, is the electric circuit completed? Are there no electrical effects in the glass? The great theory of Maxwell— the electro-magnetic theory of light-assumes that there is an electrical effect in the dielectric in the shape of what are called displacement currents, which arise when the electrical energy which is stored up in the ether near the metallic coatings of the jar is made to fluctuate in amount. The cycle of electrical effects manifested by a Leyden jar is made up, therefore, of currents of conduction along the wires connecting the coatings and of displacement currents in the glass. The study of these displacement currents, both from a mathematical and an experimental point of view, is now of the utmost theoretical importance.

The phenomena of the spark from an electrical machine or a charged Leyden jar differ from those exhibited by the voltaic are mainly in this: the spark studied by Franklin is not one spark, but each spark is made up of a number of sparks which oscillate to and fro. This was first shown by Joseph Henry, and it is a remarkable fact, the importance of which will be seen when we study electrical waves.

When we obtain a great difference of potential between the carbon terminals-for instance, when a spark from a Leyden jar jumps between them—we perceive a marked difference between these poles. A good way to study these differences is to distribute light powder, like lycopodium, on glass, bring wires from a Ruhmkorff coil to opposite points on the glass, and break the circuit of the primary. The light dust is disturbed, and a figure will result on the glass due to the state of electrification of the point of the wire. These figures are different at the positive and negative terminals of the wire, and they have been carefully studied by Bezold and other physicists. Another interesting way to study these figures is to replace the glass plate and the dust by an ordinary photographic plate. On developing the plate a photograph is obtained which shows that the discharge at either the negative or the positive pole is a complex figure made up of both the discharge peculiar to the negative pole and that peculiar to the positive pole. Now it may be asked, How do we know that the figure is a combination of these effects? What we wish to show is, that the discharge from a Leyden jar is a to-and-fro discharge that it is oscillatory, in other words. In order to prove this, it is necessary to use a revolving mirror to study the discharge. The following is the modified form of Feddersen's apparatus which I have used in my researches, and which I shall often have occasion to refer to in what follows: A little concave mirror is mounted on the armature shaft of a little electric motor, E (Fig. 25). The spark gap is placed just above a sensitive plate, P, which is shielded from the direct light of the spark. If the mirror, M, is at rest, the photograph obtained by reflection at P is simply a zigzag line. When, however, the mirror revolves very swiftly

the photograph of the spark is drawn out into a band like a comet's tail, which is seen to be made up of a series of dots of alternate degrees of brightness (Plate I, frontispiece). The darkest dots represent the stronger discharge at the positive terminal, and the light dots the

M

E

FIG. 25.

discharge at the negative terminal. The discharge oscillates to and fro until it dies out. What seems to the eye, therefore, as one spark is made of a number. The number depends upon the length of wire in the circuit between the two coatings of the jar and the

thickness and size of the jar, or, in other words, what is termed the capacity of the jar. It may be thought that a rough manner of showing the oscillatory nature of the discharge of a Leyden jar would be to perforate a piece of writing paper by the discharge; for, on examining the hole made by the spark, it will be seen to have a burr on both sides of the paper, as if the discharge had passed to and fro through it. This method, however, is an erroneous one, for if the discharge is made nonoscillatory by discharging through a wet string or a suitable liquid resistance, the burrs are also obtained. The phenomenon is therefore due to a species of explosion in the paper, and is not due to the oscillations of the spark discharge.

If we analyze an ordinary spark by the revolving mirror we discover that it is generally oscillatory. When, therefore, people imagine that they can tell which way a lightning discharge passes, whether from the clouds to the earth, or from the earth to the sky,

they must reflect upon this oscillatory phenomenon, and also consider that the interval between such oscillations is less than one ten millionth of a second. An impression remains on the retina about one sixteenth of a second, and the human eye, therefore, can not distinguish direction in the electric spark.

I have used the following method of studying the phenomena at the poles of discharge: The terminals, between which the spark jumps, consist of two thermal junctions. Immediately after the discharge occurs the circuit between the junctions is completed through a galvanometer by a peculiar key. In the case of the oscillatory discharge, the two terminals are heated equally, and there is no movement of the galvanometer needle. When, however, the revolving mirror shows that the discharge is nonoscillatory-and this can be accomplished by putting in a suitable liquid resistance in the path between the two coatings of the Leyden jar -the galvanometer shows that one junction at the positive terminal is more heated than that at the negative.

In 1842, Prof. Henry, in speaking of what was called anomalous magnetism, which was observed in the case of needles magnetized by discharges from Leyden jars these needles often exhibiting a magnetic condition opposite to that which should result from a current in a definite direction-says:

"This anomaly, which has remained so long unexplained, and which at first sight appears at variance with all our theoretical ideas of the connection of electricity and magnetism, was, after considerable study, satisfactorily referred by the author to an action of the discharge of the Leyden jar which had never before been recognised. The discharge, whatever may be its nature, is not correctly represented (employing for sim