

TELEGRAPHY, WIRELESS. 



715 



tions in it. It was then found that induction was 

 a very serious source of annoyance. In one early 

 case in England it was found, for instance, that 

 telephone wires were disturbed on the top of houses 

 80 feet high by telegraph wires running parallel to 

 them under ground. In 1885-'87 experiments near 

 Newcastle-on-Tyne, England, under the direction 

 of W. H. Preece, the head of the British Postal Tele- 

 gniph, showed that one wire was affected by dis- 

 turbances in a parallel wire a quarter of a' mile 

 distant. At Swarland Park in August, 1886, speech 

 was carried on by telephone between two parallel 

 horizontal squares of insulated wire a quarter of a 

 mile apart, the side of each square being also a quar- 

 ter of a mile. At Broomhill colliery, shortly after- 

 ward, similar communication was held between a 

 triangle of insulated wire with side of three quar- 

 ters of a mile, laid out in the underground galleries 

 of the colliery, and a similar triangle 860 feet above 

 on the surface of the ground, showing that rock was 

 not opaque to the inductive effect. In Bristol 

 Channel Mr. Preece sent messages from Penarth to 

 Flat Holm, a distance of three miles, and he ob- 

 tained evidences of sound as far as Steep Holm, 

 over five miles. His system is now in regular prac- 

 tical operation, being used by the British War De- 

 partment between Lavernock Port and the neigh- 

 boring fort on the island of Plat Holm. 



In 1894 Charles A. Stevenson communicated to 

 the Edinburgh Royal Society the results of experi- 

 ments in signaling from Burra-fiord lighthouse, 

 Scotland, to North Unst lighthouse on the island of 

 Muckle Hagga, a distance of three miles ; but of 

 this only half a mile of sea channel was worked by 

 induction, the remainder being covered by ordinary 

 conducting line wire. On each side of the half-mile 

 break was a coil of nine turns of wire, inclosing a 

 circle 200 yards in diameter. Mr. Stevenson did 

 not use alternating currents as Mr. Preece did, but 

 listened for the make-and-break ticks caused by a 

 battery and key. Mr. Stevenson states that he could 

 read by induction messages passing between Edin- 

 burgh and Glasgow ; but this is doubted by Dr. 

 Oliver Lodge. 



Simple induction telegraphy is illustrated by the 

 accompanying diagram of Preece's method (Fig. 2), 

 in which A is a source of electricity, 

 B a Morse key, C a " buzzer " of the 

 kind used in field telegraphy, D and 

 E rolls of insulated wire about 150 

 yards long, and F a telephone trum- 

 pet. In practice, of course, the coils 

 are strung out, and instead of being 

 close together are half a mile or so 

 apart. 



Messrs. Evershed and Cox have de- 

 vised a receiver consisting of a vibrat- 

 ing loop of wire tuned to the sending 

 circuit, and suspended in a powerful 

 magnetic field. This is said to be very 

 sensitive. With this they have made 

 important experiments in sending sig- 

 nals to lightships off the coast. 



Dr. Oliver Lodge, who has made a F 



close study of the whole subject, has 

 developed a system of magnetic-in- 

 duction telegraphy in which he uses at both the 

 sending and the receiving station a large hori- 

 zontal coil of wire, including a condenser of con- 

 siderable capacity. These circuits are syntoni/.ed 

 or "tuned" so that each has the same natural 

 electric vibration frequency, and hence a disturb- 

 ance in one acts by the principle of resonance to 

 cause a much more intense disturbance in the other 

 than would be caused by simple induction. The 

 frequency is lowered to that of sound, so that <i 

 telephone can be used at the receiver to detect the 



ili-i urbance. Some alternating or intermitting ma- 

 chine of considerable power is used as the sending 

 appliance; and its fluctuating current is broken 

 into long and short "spells, so that the Morse 

 alphabet can be used for signaling. I)r. I^xlgc 

 has also sometimes dispensed with the condensers, 

 and used the coils alone; but the results wen n..t MI 

 satisfactory. Dr. Lodge believes that he has shown 

 mathematically that "no unaided simple induction 

 process can work satisfactorily over really big di 

 tances, unless an altogether prohibitive amount >f 

 wire or an extravagant amount of power is u- 

 The hearing distance with simple induction is as 

 the two-thirds power of the diameter of one of the 

 coils: so that both coils must be nearly doubled if 

 the distance is to be doubled. 



There are, as has been seen, two practicable ways 

 of building the inductive circuits in this variety c,f 

 space telegraphy; in that devised by Preece they 

 are in vertical planes, and consist half of line-wire 

 and half of the return-earth circuit. In Lodge's 

 method they are horizontal. In horizontal circuits 

 there is a loss of one half the mutual induction, 

 which, however, can be compensated for by in- 

 creasing the area, and more wire is necessary." On 

 the other hand, horizontal circuits, especially in the 

 form of coils, possess manifest advantages. " In 

 every instance in which communication has been 

 carried on by magnetic induction, or by earth leak- 

 age, the ordinary Bell telephone has been used as a 

 'sounder.' It is well adapted for the purpose, as it 

 enables exceedingly minute currents to be detected. 

 This sensitiveness does not arise so much from the 

 efficiency of the telephone as a current detector as 

 from the marvelous delicacy of the human ear, 

 which is able to distinguish sound waves produced 

 by motions so exceedingly small as to be absolutely 

 invisible even when magnified many times." (S. 

 Evershed, Inst. Elec. Engineers, Dec. 22.) Mr. S. 

 Evershed has made careful measurements of the 

 magnetic and electrical quantities involved in the 

 telephone, but up to the present he reports that he 

 has been unable to devise adequate means for meas- 

 uring what fraction of the electrical power supplied 

 is converted into mechanical power. This conver- 

 sion can be estimated for any given amplitude of 

 the diaphragm from a knowledge of the change 

 in induction threading the coils for different dis- 

 placements of the diaphragm; but the difficulty 



FIG. 2. MR. PREECE'S INDUCTION METHOD. 



remains of determining the amplitude when an 

 alternate current is flowing in the roils. 



In all the induction experiments that have been 

 described, the message was carried through space 

 from transmitter to receiver by low-frequency elec- 

 tric waves. In the method now to be described 

 high-frequency Hert/ian waves are employed. 



Telegraphy by Hertzian Waves. The CDMOT- 

 ery of electric waves by lleinrich Hertz (see yearly 

 articles on PHYSICS in "Annual Cyclopaedia ") 

 gave a great impulse to wireless telegraphy. Every 



