TELEGRAPHY ACROSS SPACE. 239 



system was successful both for telegraphing and telephoning, and was 

 indeed adopted for a time by the Lehigh Valley Railroad Company. 

 But it has been abandoned for a very simple reason. One of the 

 consolations of railway traveling is that one is free from being disturbed 

 by telegraph or telephone. No one on board an express wants to 

 telegraph or to be telegraphed to. 



Electro- magnetic induction has played so an important a part in dis- 

 tance telegraphy that it must receive a more extended notice. Very 

 early after the introduction of the commercial telephone, troubles arose 

 from the exceeding sensitiveness of the instrument. Conversations in 

 one line were overheard in another; while the ear was disturbed by an 

 incessant buzz, or rattle, from the interference of stray currents from 

 neighboring telegraph lines. All these were at first attributed to in- 

 duction; that is to say, to the electro-magnetic influence of the currents 

 in one line upon the neighboring line. No doubt, in some cases, this is 

 a cause; but unquestionably, in many of the cases, the disturbance 

 was due not to induction at all, but either to leakage of currents across 

 the surfaces of the insulating supports, over films of dirt or moisture, or 

 else to leakage of currents from one line into the other through the 

 earth plates or earth connections. Unless circuits with metallic returns 

 are used, it is certain that the earth return will afford a means for 

 stray currents to find their way into the telephone lines. Mr. Preece 

 has narrated many cases in which telegraph or telephone messages 

 that are being transmitted along some line have been heard, or rather 

 overheard, in telephonic instruments in some totally disconnected and 

 distant line. Many of these are due doubtless to stray currents 

 through earth; but some are unquestionably due to true induction. 

 A line or circuit absolutely insulated from any earth contact or earth 

 return may yet act inductively. During the brief instant while the 

 current in it is growing, that current is setting up a magnetic field in 

 the surrounding region, extending indefinitely, but feebly, into space. 

 As the current dies away again, this magnetic field also dies away. If 

 in its growth or decrease, this magnetic field encounters other wires, 

 it sets up electro-motive forces in them, and thus originates disturb- 

 ances. For the propagation of this effect from wire to wire no contact 

 is needed. It is an effect that is dependent upon the properties of the 

 intervening medium, and is proportional to its magnetic permeability. 

 The ether of space itself, air, water, soil, and rock, all are of about 

 equal permeability. Hence this kind of induction may be propagated 

 from circuit to circuit, whatever natural material intervenes. Mr. 

 Preece has made repeated researches with the view of utilizing this 

 effect for the purpose of distance telegraphy. He has erected parallel 

 base lines, sometimes in South Wales, sometimes near the mouth of 

 the Dee, sometimes in Scotland. He has laid out, flat on the ground, 

 great squares of iusulated wire, to test the inductive transmission from 

 one area to another. On Newcastle town moor, and on the sands at 



