184 The N.Z. Journal of Science and Technology. [Aug. 
electric-railway lines parallel for considerable distances the land telegraph- 
lines, electric disurbances are frequently induced in the latter to such an 
extent as to render them temporarily inoperative or to require the per¬ 
manent employment of special, and in many cases expensive, methods for 
their neutralization. 
Telegraph-lines—by reason of the fact that they are operated upon 
the earth-working principle, i.e., use the earth as the return circuit—are 
inherently susceptible to disturbances by any natural electrical phenomena 
capable of varying within certain limits the potential of the earth—such, 
for example, as magnetic storms. Without discussing the causes under¬ 
lying such phenomena, it may be stated that their principal and practical 
effect upon a telegraph-line is to cause differences of electrical potential 
at different points of the earth’s surface, and so to generate varying 
electrical currents in wires that may be connected therewith. These 
currents are occasionally of such magnitude that they overpower the 
legitimate signals, and render telegraph circuits unworkable on the earth- 
return principle. Recourse has then to be had to the metallic system— 
i.e., to the substitution of a wire for the earth return. 
Telephone-lines. 
Telephone circuits are worked mainly upon the metallic-circuit system, 
which employs two wires instead of using the earth as a return conductor. 
These circuits are practically immune from disturbance due to natural 
phenomena of the kind referred to, as they are not in any way connected 
to the earth, and cannot, therefore, reflect the variations of electrical 
potential produced therein. The use of two wires and the special methods 
adopted in their erection, whereby they are relatively equidistant from other 
circuits either on the same pole line or on adjacent power-lines, serve also 
to render the circuit immune from the influence of the electric and magnetic 
fields of neighbouring electric-power and tramway circuits, which would 
otherwise, by reason of induced noises, make the telephone circuits unsatis¬ 
factory for the transmission of speech. There are, however, in most countries 
a number of long-distance earth-working telephone-lines connecting up 
remote centres with the larger cities. Such circuits are subject to disturb¬ 
ance by extraneous electrical circuits, and on occasions give unmistakable 
evidence of the variation in the potential of the earth caused by natural 
phenomena, and even of variation in the earth’s magnetic field. These 
latter disturbances manifest themselves at odd times by chirps and uncanny 
noises in the telephone receiver ; but they do not usually affect to an 
appreciable extent the commercial operation of the circuit. 
Submarine Cables. 
Submarine cables are almost exclusively of the earth-working type, 
and use comparatively delicate receiving-apparatus ; they are therefore 
affected in a large measure by electric and magnetic phenomena. Apart 
from special disturbances such as the aurora, “ earth currents ” are 
practically always present and are a permanent source of disturbance, the 
effects of which have to be compensated for in various ways. When 
periodical tests of submarine cables are made— e.g., across Cook Strait— 
these earth currents interfere with the testing-apparatus, and in order that 
reliable results may be obtained numerous devices and calculations have 
to be resorted to in order to neutralize or compensate for the erroneous 
effects produced by these stray earth currents. Even the Wellington 
