Waves and ripples in water, air, and æther

by Sir J. A. Fleming

The human body consists largely of water which exists in the tissues, and hence it is not surprising to find that the hand or any part of the body placed between the radiator and receiver intercepts the electric ray. You see, if I hold my hand in front of the radiator, that nothing is able to escape from it, when sparks are made between the balls, which can affect the receiver. In the same way it can be shown by experiment that the human head is perfectly opaque—in fact, much more opaque than an equally thick block of wood; and this opacity to the electric ray is due in a veritable sense to the water in the brain. All dry animal tissues, such as leather, bone, gelatine, and flesh, if dry, are very transparent to electric radiation of the kind we are now using, but if these objects are made thoroughly wet, then they become intensely opaque.

Fig. 76.—The reflection of an electric ray.

We can, then, proceed to show that this electric radiation can be reflected, just like light or sound, by metal or other conducting surfaces, and that the law of reflection of the electric ray is the same as the law of reflection for rays of light or sound. If we place the radiator A with its mouth upwards, still preserving the receiver B in a horizontal position, it is possible to adjust the two very near to one another, but yet so that the radiation from the radiator does not affect the receiver. If I now hold a metal plate, P, at an angle of 45° above the mouth of the radiator, you will notice that the bell at once rings, thus showing that the electric radiation has been reflected into the receiver-box ([see Fig. 76]). Also we find that a very small deviation from the angle of 45° is sufficient to prevent the effect. Careful experiments in the laboratory show that the electric ray is reflected according to the optical law, viz. that the angle of reflection is equal to the angle of incidence. We find that any good conducting surface will, in this manner, affect the electric radiation. Thus I can reflect it from a sheet of tinfoil or even from my hand, and the fact that I can, so to speak, take hold of this electric radiation, and deflect it in different directions by the palm of my hand, produces in the mind a very strong conviction that we are dealing with something of a very real nature in experimenting with this electric radiation.

It will be in your remembrance that, in the chapter in which we were dealing with waves in the air, I showed you a very interesting experiment illustrating the refraction of rays of sound by means of a carbonic acid prism, and I have now to bring before you an exactly analogous experiment performed with electric radiation. Here, for instance, is a prism made of paraffin wax, a substance which you have already seen is transparent to the electric ray. If we arrange the radiator- and receiver-boxes at an angle to one another, it is possible so to adjust them that the electric radiation projected from the radiator-box A just escapes the receiver-box B, and does not therefore cause the bell to ring ([see Fig. 77]). When this adjustment has been made we introduce the paraffin prism P into the path of the electric ray, and if the adjustments are properly made, we find that the electric ray is bent round or refracted, and that it then enters the receiver-box and causes the bell to ring. This experiment was first performed by Hertz with a very large pitch prism, but his apparatus was too cumbersome for lecture purposes, and the smaller and more compact arrangement you see before you is therefore preferable for present purposes.

Fig. 77.—The refraction of an electric ray.

I have it in my power to show you a still more remarkable experiment in electric refraction. It is found that dry ice is very transparent to these electric rays, but if the ice is wetted on the surface, then, as you have already learnt, the film of moisture is opaque. We have had constructed for the purposes of this lecture a prism of ice by freezing water in a properly shaped zinc box. This prism is now being arranged between the radiator and the receiver, and its surfaces must next be dried carefully with dusters and white blotting paper to remove every trace of moisture. When this is done we find we can repeat with the ice prism the same experiment performed just now with the paraffin prism, and we can refract the electric ray. If you will recall to your memory the statements which were made in connection with the refraction of rays of sound and waves of water, you will remember that it was pointed out that the refraction of a ray of sound and the bending of a train of water waves was due to the passage of the waves in the air or in the water from a region where they were moving quickly to a region in which they were compelled to move more slowly; and it was furthermore shown that this bending must take place whenever a plain wave of any kind passes in an oblique direction from one region to another region where it undergoes an alteration in velocity. In other words, it was shown that the bending or refraction of the direction of motion of a wave, whether in air or water, is a proof that there is a difference in its velocity in the two places bounded by the surface at which the refraction takes place. If this bending takes place in such fashion that the ray is bent towards the perpendicular line drawn to the bounding surface, which is the same thing as saying if the line of the wave is bent so as to make a less angle with the bounding surface after it has passed from one region to the other, then it shows that the wave-motion travels more slowly after it has passed the bounding surface than before.

If we now return to the consideration of the electric experiment with the prism of paraffin or ice, we shall find that this, properly interpreted, gives us a proof that the electric radiation travels more slowly in paraffin wax or ice than it does in air, and the ratio between its velocity in air or in empty space and its velocity in any non-conductor is called the electric index of refraction for that non-conductor. This index can be determined by making two measurements. First, that of the refracting angle of the prism; and secondly, that of the deviation of the ray.[26] I have made these two experiments for the prisms of paraffin and ice in my laboratory, and I find the electric refractive index of paraffin to be 1·64, and the electric refractive index of ice to be 1·83.

In connection with the refraction of rays of sound, it was pointed out that a curved surface has the power to diverge or converge rays of sound, and you will remember that we employed a sound-lens for converging the rays of sound diverging from a whistle, just as an ordinary burning-glass, or double convex lens, can be employed to bring the rays of sunlight to a focus. We shall now attempt a similar experiment with the electric ray. A block of paraffin is fashioned into the shape of a semi-cylinder, flat on one side and convex on the other, and this plano-convex paraffin lens has a convex surface having a radius of 6 inches. If I place the radiator A and receiver B about 4 feet apart, then by making a few little adjustments it is possible to so arrange matters that the radiation which proceeds from the radiator is not powerful enough at a distance of 4 feet to sensibly affect the coherer and make the bell ring ([see Fig. 78]). If, however, I adjust the paraffin lens L halfway between, I shall converge this electric radiation to a focus just about the place where the coherer is situated, and the consequence is that on making sparks between the balls of the radiator, we find that the bell attached to the receiver at once rings.