Air Conductor.
The vertical wire extending from the coherer up into the air must be insulated from all other objects in the best possible manner. A bare copper wire of No. 14 B & S gauge can be suspended from porcelain insulating knobs, which in turn can be strung from each other by means of stout silk cord or even wire. There is a special form of insulator used in electric construction work, and known as a circuit breaker, which will answer and which is easy of attachment; reference to Fig. 79 will show manner of using.
Fig. 79.
Temporary grounds can be made to water pipes, but it is better to use regular telephone copper ground-plates sunk deep in moist earth.
At South Foreland, England, a mast has been erected, 150 feet in height for transmission across Channel, a distance of nearly thirty miles. At Notre Dame University, Illinois, Professor Green used a wire 150 feet in length, suspended from top of a high church tower, but was unable to transmit much over three miles, owing, presumably, to fact that the intervening country was well supplied with overhead wires, which probably intercepted the waves.
It has been claimed that earthed or grounded air wires are necessary, but balls or similar "capacities" are not of service on the top of the wire. A theory has been advanced that the currents do not pass from air wire tip to air wire tip, but are conducted by the varying strata of the earth. No general confirmation is obtainable, however, and the experimental reader will find a wide field for research in this direction. Marconi, on the other hand, has accomplished much with zinc cylinders under six feet high, not grounded in any respect, indeed, and he also finds it impossible to assume a proportion between distance of effect and height of air wire. The following investigations and experiments are of interest in this connection:
At a meeting of the Institution of Electrical Engineers, in December, 1898, Dr. Oliver Lodge showed that there must be a certain relative position between the receiving and transmitting circuits.
He placed on one side of a room a box, containing a battery, bell, relay, and coherer properly connected up. On the other side he had an induction coil and pair of parallel discharger rods, with a spark gap to transmit waves across the room. When the rods of the receiver and transmitter were placed parallel to each other the receiving bell was operated; when the receiving rods of the transmitter were at right angles to those of the receiver the bell either failed to work, or weakened very considerably. He also told of an experiment made to determine the influence of different methods of grounding the apparatus. He found that when the apparatus was connected by a wire laid on the ground, there was the required response at the receiving station; but when the two stations were situated each side of a lake, and the ground wires immersed in the water, the receiving instrument failed to work. It seemed to him that the conductivity and power absorption of ether wave energy by water was too great to allow of the transmission of Hertz waves. This would seem to bear out the results obtained by Marconi in dispensing with ground wires.