Translating Devices.
Under this head are included relay sounder, bell, or register, which are at receiving set. They do not differ from the regular telegraphic apparatus. The sounder may be of the Western Union pattern, wound to 4 ohms; the relay also Western Union pattern, and wound to 150 or 250 ohms, as best suits the individual case.
In order to protect the receiver from the action of the transmitter belonging to the same set of instruments, particularly when powerful waves are generated, it has been found at times necessary to enclose the receiver in a metal case. Marconi has patents on such devices, particularly on a movable shutter in the case, which opens when the transmitter is not in operation. Edouard Branly placed his receiving set in a metal case with a vertical slit eight inches by one-tenth of an inch.
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.