Let us examine the sending spark a little further. An electric spark is perhaps the most interesting phenomenon in electricity. What causes it--how does the air behave toward it--what is it that apparently flows through the air, sending out light and heat waves as well as magnetic and electric waves? If we could answer all these questions, we should know what electricity is. A critical study of the electric spark has not only its scientific but its practical side. We see the latter side evidenced by its employment in wireless telegraphy and in the X rays; for in the latter case we have an electric discharge in a tube from which the air is removed--a special case of an electric spark. In order to understand the capabilities of wireless telegraphy we must turn to the scientific study of the electric spark; for its practical employment resides largely in its strength, in its frequency in its position, and in its power to make the air a conductor for electricity. All these points are involved in wireless telegraphy. How, then, shall we study the electric spark? The eye sees only an instantaneous flash following a devious path. It can not tell in what direction a spark flies (a flash of lightning, for instance), or indeed whether it has a direction. There is probably no commoner fallacy mankind entertains than the belief that the direction of lightning, or any electric spark, can be ascertained by the eye--that is, the direction from the sky to the earth or from the earth to the sky. I have repeatedly tested numbers of students in regard to this question, employing sparks four to six feet in length, taking precautions in regard to the concealment of the directions in which I charged the poles of the charging batteries, and I have never found a consensus of opinion in regard to directions. The ordinary photograph, too, reveals no more than the eye can see--a brilliant, devious line or a flaming discharge. [Illustration: FIG. 7.--Arrangement of batteries of motor (to disturb the coherer) and the sounder by which the messages are received.] A large storage battery forms the best means of studying electric sparks, for with it one can run the entire gamut of this phenomenon--from the flaming discharge which we see in the arc light on the street to the crackling spark we employ in wireless telegraphy, and the more powerful discharges of six or more feet in length which closely resemble lightning discharges. A critical study of this gamut throws considerable light on the problem of the possibility of secret wireless telegraphy--a problem which it is most important to solve if the system is to be made practical; for at present the message spreads out from the sending spark in great circular ripples in all directions, and may be received by any one. [Illustration: FIG. 8.--Photograph of electrical pulses. The interval between the pulses is one millionth of a second.]