HIGH RESISTANCES.

145

stroyed it. This purpose is effected by introducing into the circuit at the places marked p1 p2, q, s (in Fig. 39), high resistances having as little self-induction as possible. The action of the high resistances is that, while preventing an appreciable quantity of the current from passing through them when the apparatus is working, they nevertheless afford an easy path for the currents of high tension which would be formed at the mount when the circuit was broken, and

P2

FIG. 40.

thus prevents sparking at contacts or sudden jerks of currents, which would restore or maintain the conductivity of the sensitive tube. Similar resistances or condensers are provided at other points for the like purpose, as, for instance, to prevent the high resistance oscillations set up across the plates of the receiver by the transmitting instrument, which should pass through the sensitive tube, from running round the local battery wires and thereby weakening their effect on the sensitive tube or contact.

Fig. 40 represents a complete transmitting and receiving station. When working the apparatus it is necessary either that the local transmitter and receiver at each station should be at a considerable distance from each other, or else that they should be screened the one from the other by metal plates. The usual method is to have the apparatus, with the exception of the sending key and the reading instrument, enclosed in a metal box which is connected to earth.

CHAPTER X

Marconi's first experiments in England-Trials on the Bristol Channel-Also between the Needles and Bournemouth-Experiments at Spezia-Valuable results obtained-Professor Slaby's investigations at Potsdam and elsewhere-Further experiments by Marconi-Wireless telegraphy on board the Royal Yacht Aerial communications between England and France-British and French Associations for the Advancement of Science-Wireless telegraphy at the Naval Maneuvers-Experiments of the Brothers Lacarme-Communication with balloons-Trials by the United States Navy Board, etc.

MARCONI's first experiments in England took place in the General Post-Office building itself, under the supervision and with the able assistance of Sir William Preece. These having proved successful, his system was submitted to a more critical test on Salisbury Plain, with a clear distance of two miles between the sending and receiving stations. In these experiments parabolic reflectors and resonance plates were used.

These trials having proved successful, Marconi's apparatus were subjected to a more searching trial, along with Preece's own method. The experiments took place between Lavernock Point and Flatholm (3.3 miles), and also between

Lavernock Point and Brean Down (8.7 miles) on the opposite side of the Bristol Channel. Here the reflectors were done away with and vertical wires employed in their stead. The receiving station was fixed at Lavernock Point, twenty yards above the level of the sea. A mast thirty yards high was erected and on the top of it placed a cylindrical cap made of zinc, two yards long and one in diameter. Connected with this cap was a copper wire leading to one electrode of the coherer, the other electrode being attached to a wire that descended into the sea.

The sending apparatus was placed on Flatholm where the vertical wire and the zinc cap resembled those at Lavernock Point. A Ruhmkorff coil giving twenty-one inch sparks, with an eight-cell battery, was used for generating the Hertzian waves. After some experiments had been made with Preece's method (already described), which were entirely successful, Marconi's apparatus was put to the test. At first the trials were anything but satisfactory-indeed they were little short of utter failures. Next day, however, May 12, the vertical wire having been lengthened by twenty yards, the results, though still unsatisfactory, were better; while on the 13th, when the receiving apparatus was taken down from the cliff to the beach and a further length of wire added, the success achieved was beyond doubt.

TRIALS AT SPEZIA.

149

The experiments which followed betwixt Lavernock Point and Brean Down were equally satisfactory, as were also similar trials that took place in the following November between the Needles, Alum Bay, Isle of Wight, and Madeira House, Bournemouth.

These experiments attracted so much attention that the Italian Ministries of War and Marine caused a series of trials to be made at Spezia between July 11 and July 18, 1897, under Marconi's direction. The first three days were devoted to trials on land, when excellent results were obtained at a distance of 3.6 kilometers. On the 14th the scene of operations was transferred to the water. The sending apparatus, which was installed in a tent upon a tongue of land near the arsenal of St. Bartholomew on the eastern side of the Gulf of Spezia, consisted of an oscillator with two central spheres of ten centimeters and two outer spheres of five centimeters diameter and an induction coil with sparks twenty-five centimeters in length, supplied by an accumulator battery. The vertical wire was twenty-six yards in length and terminated in a zinc plate.

The receiving apparatus was set up on a tugboat, and had a vertical wire running to the top of a mast sixteen yards high and terminating in a zinc plate, while another wire led from the coherer into the water. Transmission was suc