which the wave motion was propagated. This method of procedure is of great importance; for we have seen that the early methods of measurement of the velocity of electricity were vitiated, so to speak, by the conditions of the apparatus which was used for the measurement, and later calculations depend upon estimations of selfinduction and of the time of the periodic motion of a circuit, which is not the same as that along which the motion takes place. In short, our measure is a direct measure of the speed of the electric waves in the ether near the surface of the wires we employed.

The arrangement and dimensions of the apparatus finally adopted were as follows (see Fig. 45, page 246):

Two metallic plates, a and b, 30 X 30 centimetres, placed in vertical planes, formed the primary condenser. The dielectric between them consisted of the best French plate glass obtainable, and was two centimetres thick. Outside the plates a and b, and separated from them by a hard-rubber dielectric, 1.8 centimetre thick were the secondary plates 26 x 26 centimetres. The primary and secondary circuits were joined to the condenser plates as indicated in the figure. The primary circuit lay in the horizontal plane passing through the centres of condenser plates, and consisted of copper wires, 0.34 centimetre in diameter. In order to control the period of oscillation of the primary circuit, the portion BD, containing a spark gap with spherical terminals, was made to slide along parallel to itself. The distance between the straight portions A B and C D was 40 centimetres, and the lengths of A B and C D finally chosen for best resonance were 85 centimetres. Most of the secondary circuit lay in a horizontal plane 15 centimetres above that of the primary. The lengths GE and HF, however, were bent down and fastened

to the middle points, G and H, of the secondary plates. The circuit consisted of copper wire (diameter 0.215 centimetre), and its total length from G through I to H was 5,860 centimetres. At I was a spark gap with pointed terminals. With this apparatus we succeeded in producing a very regular wave formation, as indicated by the bolometer. There was a node at I, and another about 40 centimetres to the right of E and F.

The images of the secondary spark were thrown on a sensitive plate by means of a rotating mirror. The dots obtained represent discharges from the negative terminals only, the positive discharges not being brilliant enough to affect the plate. The distance between successive dots was the distance on the plate through which the image of the spark gap moved during the time of a complete oscillation. Hence by determining the speed of the mirror and measuring the distances from the mirror to the plate the time of oscillation could be calculated. To measure the sparks, we used a sharp pointer moved at the end of a micrometer screw, under a magnifying glass of low power. The instrument was originally intended for microscopic measurements, and was very accurately constructed. The rotating mirror was driven by an electric motor by means of a current from a storage battery of extremely constant voltage. To give great steadiness, a heavy fly wheel was attached to the axis of the mirror. The speed of the mirror was determined to within about one part in five hundred by means of an electric chronograph.

It appears from the best results that we have obtained that the velocity of short electric waves travelling along two parallel wires differs from the velocity of light by less than two per cent of its value. It has been shown theoretically that the velocity of such

waves travelling along a single wire should be the velocity of light approximately. These results, therefore, in a certain sense, confirm the theory, to an accuracy within their probable error.

Theoretically, too, the velocity should be approximately equal to the ratio between the two systems of electrical units. The average of the best measurements of this ratio is 3·001, which is nearer the average velocity obtained for electric waves than the velocity of light.

We have established, I believe, beyond reasonable doubt, that the waves of electricity travel with the velocity of light, for the waves on the wires we employed must have been proved to reside entirely on the surface —that is, on the boundary, so to speak, of the medium pervading the space about the wires. The formation of stationary electric waves which are propagated with the velocity of light is the best evidence we now have of the truth of Maxwell's great generalization.

We have said that Joseph Henry showed at an early date that the discharge of a Leyden jar is oscillatory. Our present knowledge of electric waves is largely due to a realizing sense of the importance of the observations of Henry. One will find in his published paper the following remarkable conclusion, which can be regarded almost as a prophecy:

"In extending the researches relative to this part of the investigations, a remarkable result was obtained in regard to the distance at which induction effects are produced by a very small quantity of electricity. A single spark from the prime conductor of a machine of about an inch long, thrown on to the end of a circuit of wire in an upper room, produced an induction sufficiently powerful to magnetize needles in a parallel cir

cuit of iron placed in the cellar beneath, at a perpendicular distance of 30 feet, with two floors and ceilings, each 14 inches thick, intervening. The author is disposed to adopt the hypothesis of an electrical plenum, and from the foregoing experiments it would appear that a single spark is sufficient to disturb perceptibly the electricity of space throughout at least a cube of 400,000 feet of capacity. And when it is considered that the magnetism of the needle is the result of the difference of two actions, it may be further inferred that the diffusion of motion in this case is almost comparable with that of a spark from a flint and steel in the case of light."*

* Scientific Writings of Joseph Henry, vol. i, p. 202, Smithsonian Institution, Washington.

CHAPTER XIX.

THE ELECTRO-MAGNETIC THEORY OF LIGHT AND

THE ETHER.

THE various phenomena of the action between magnets the induction phenomena between neighbouring circuits, a current of induction rising in one circuit whenever an electric current is started in a neighbouring circuit, and thus manifesting energy-lead us to believe that the energy has been transferred from the exciting circuit across space by means of some medium filling that space. The energy has disappeared from the exciting circuit, and has reappeared in the induction circuit. It must have existed during the time of its disappearance and reappearance in the intervening space. We are therefore forced to believe in some medium which serves to convey this energy.

The old fluid theories implied that when a body was electrified it had something upon it which was called electricity. According to the modern views, we regard the ether around the body as charged with energy which is the result of the work we have done in charging the body. This energy in the ether is the energy of motion. There is a state of strain in the ether which we term a polarized condition. Around a positively charged body this polarization has a certain direction and a certain amount. With a negatively