*ELECTRICAL WAVES.*

and, if now the Ruhmkorff be excited, a series of stationary electrical waves will be formed in the wire. To detect these we employ the principle of resonance. A wire whose time of oscillation has been determined and found to be nearly equal to that of the primary conductor is bent into a circle, and the ends are brought close together. This is then brought close to the long wire, and held so that its plane embraces the latter. A fine display of sparks will be seen to accompany the Ruhmkorff discharge.

If this proof circuit be approached to the extreme end of the long wire, no sparks will be seen. The wire has at its end, in fact, a node the same as a stopped organ-pipe has. As the air in the pipe is undisturbed, so the potential of the wire end is unchangeable. As we recede from the end, the sparks grow longer, but finally disappear again. Here is another node. We measure the distance between the two and cut the wire so that its total length shall be a multiple of this length, and then we proceed to find all the nodes, and mark them by paper riders. If we measure each of these distances and take the mean, or measure the whole length of the wire and divide by the number of nodes, we have a value for the wave-length of the conductor. In Hertz's experiment this value was 2ยท8 metres. From this value, and the time of oscillation reckoned from the self-induction and capacity, he gets the velocity of propagation of electrical disturbances as two hundred thousand kilometres per second. This result Hertz prints in bold-faced type, and puts it as a climax of all his work. This is truly wonderful. If we consider that the calculated value of the time of oscillation depends upon the assumption that the velocity of electrical wave propagation is the same as that of light (three hundred thousand kilometres per second), and this circuitous calculation of the same thing gives two hundred thousand kilometres per second, we can hardly give Hertz the credit of extremely accurate work. However, Hertz has made a great advance in physical science. Since Weber introduced the absolute system of units, no great advance has been made. Physicists have busied themselves in measuring the various constants, in refining and perfecting the methods of measurement, or in applying principles already known to technical and practical purposes. Hertz, however, has opened a new and unexplored field, which must eventually bring us into a closer acquaintance with the mysteries which we are daily manipulating.

This series of experiments has excited a great deal of attention in English physical circles. Prof. Fitzgerald, of this department of the British Association, laid great emphasis, at the last meeting, on the advance which had been made. Oliver Heaviside has justified his patronymic by publishing a complex mass of mathematical formulae on the subject. He considers that the waves of