reach there on their way to the bridge will always be of the opposite sign to those which reach this point after reflection, and hence the two sets of tubes will neutralise each other. At other points the two sets of tubes will always be in the same sense, and will therefore produce a loop. The experiment is exactly analogous to Kundt's method of determining the velocity of sound in gases, as described in § 314; the oscillator here corresponding to the vibrating rod which in the acoustical experiment produces the vibrations of the gas in the tube. The bridge here corresponds to the closed end of the tube, for in both cases they correspond to a node, that is, in the one case to a point where the movement of the air is a minimum, and in the other to a point where the electrical displacement is a minimum. The electrical displacement at the bridge is a minimum, for there the potential of the two wires is always the same, so that there cannot be any electro-static force, and hence no displacement. Also, just as in Kundt's experiment, the stationary waves set up in the tube are very much more intense if the length is adjusted so that it is a multiple of the half wave-length of the note given by the rod in the gas, so in the electrical case the amplitude of the stationary waves set up in the wires is much increased if the position of the bridge is altered till the length of each wire is some multiple of the half wave-length of the electrical waves in the wires produced by the oscillator.

The position of the loops on the wires can be determined by placing a Geissler tube, L (§ 381), across the wires as a bridge. If the tube is moved along it will glow brightly at the loops, but will be dark at the nodes. In this way the wave-length of the oscillations in the wires can be measured. Then, if the periodic time of the primary oscillations is known, the velocity with which the oscillations travel, when conducted in this way along wires, can be calculated. By this method, as well as by a direct comparison between the velocity in free air with that along a wire, it has been proved that the velocity is the same when the waves are propagated in a free dielectric as when they are conducted along a wire. Also, it is found that the velocity, while it is independent of the material of the wires, depends on the specific inductive capacity of the dielectric which surrounds the wires. This result is a conclusive proof that the energy travels, not along the wires, but through the dielectric which surrounds them, as is indicated by Maxwell's theory.

589. Telegraphy without Connecting Wires.-Within the last year or two much attention has been devoted to the employment of electro-magnetic waves as a means of transmitting signals from one place to another without the necessity for a metallic wire connecting the two. There are a number of arrangements which have been tried, and the whole subject is still (1899) in a very experimental stage. Some of the most successful attempts have been made by Marconi, who has transmitted signals from Alum Bay to Bournemouth, a distance of eighteen miles. As transmitter he uses an oscillator of Righi's form, giving waves of

about 120 cm. wave-length. As a receiver he uses a coherer, which is placed in series with a relay; this relay working a sounder on which the Morse signals are received. In order to make the coherer lose its conductivity when the waves stop, an electro-magnet works a small hammer which is continually tapping the tube. In the case of transmission over distances greater than a mile or two, a collector, consisting of a vertical wire attached to a pole, is used to conduct the waves to the coherer.