Several methods enable us to transform electrical energy so as to obtain suitable quick and intense blows on the surrounding medium. Is it possible that there is some mysterious vibration in the spark which is instrumental in the effective transmission of electrical energy across space? If the spark should vibrate or oscillate to and fro faster than sixteen times a second the human eye could not detect such oscillations; for an impression remains on the eye one sixteenth of a second, and subsequent ones separated by intervals shorter than a sixteenth would mingle together and could not be separated. The only way to ascertain whether the spark is oscillatory, or whether it is not one spark, as it appears to the eye, but a number of to-and-fro impulses, is to photograph it by a rapidly revolving mirror. The principle is similar to that of the biograph or the vitoscope, in which the quick to-and-fro motions of the spark are received on a sensitive film, which is in rapid motion. One terminal of the spark gap, the positive terminal so called, is always brighter than the other. Hence, if the sensitive film is moved at right angles to the path of the discharge, we shall get a row of dots which are the images of the brighter terminal, and these dots occur alternately first on one terminal and then on the other, showing that the discharge oscillates--that is, leaps in one discharge (which seems but one to the eye) many times in a hundred thousandth of a second. In practice it is found better to make an image of the spark move across the sensitive film instead of moving the film. This is accomplished by the same method that a boy uses in flashing sunlight by means of a mirror. The faster the mirror moves the faster moves the image of the light. In this way a speed of a millionth of a second can be attained. In this case the distance between the dots on the film may be one tenth of an inch, sufficient to separate them to the eye. The photograph of electric sparks (Fig. 8) was taken in this manner. The distance between any two bright spots in the trail of the photographic images represents the time of the electric oscillation or the time of the magnetic pulse or wave which is sent out from the spark, and which will cause a distant circuit to respond by a similar oscillation. [Illustration: FIG. 9.--Photograph of a pilot spark, which is the principal factor in the method of wireless telegraphy.]