420 
POPTJLAE SCIENCE KEYIEW. 
changes at one end of the cable are made to produce intelli- 
gible signals at the other end. 
We have already seen how the current of a galvanic battery 
at Valentia can be conveyed along the copper conductor of 
the cable to Newfoundland. And, of course, the current 
from a battery in Newfoundland can be conveyed in an exactly 
similar way to Valentia. 
When a current is sent through the wire, no perceptible 
effect is produced at the arrival end, so long as that end is 
in direct communication with the ground • but by connecting 
the extremity of the wire with a suitable apparatus, through 
which the current is obliged to pass before it can reach the 
ground and so return to the other pole of the battery, 
evidence of the passage of the current can be obtained by the 
production of any of its characteristic effects, such as the 
deflection of a magnetic needle or the magnetisation of soft 
iron. By breaking the connection between the battery and 
the sending end of the wire, the current can be arrested at 
pleasure, and with it the effects produced at the distant end ; 
while by re-establishing the connection these effects can be 
reproduced. This, as is well known, is the general principle 
of all processes of electrical signalling. 
In the case of long submarine lines of telegraph, such as the 
Atlantic, the effect which it is practically easiest and most 
convenient to cause the current to produce at the distant end, 
is the deflection of a magnetic needle. The apparatus 
employed for this purpose in connection with the Atlantic 
cable is Professor William Thomsons “ Reflecting Galvano- 
meter/'’ precisely the "same instrument that is used in scientific 
investigations as affording the most delicate means that we 
possess of detecting and comparing very weak electric currents. 
These galvanometers are excellently made by Messrs. Elliott 
Brothers, of London, and by Mr. James White, Philosophical 
Instrument Maker to the University of Glasgow. A beautiful 
instrument by the former makers is represented in figs. 1, 2, 
and 8 (Plate XIX.), which will enable us to explain the prin- 
ciples of its construction and mode of action. At A and B 
(fig. 1), are seen the round fronts of two rather flat metallic 
bobbins, upon which many hundred turns of very fine copper 
wire, carefully insulated throughout its whole length, are 
wound. A vertical section, parallel to the axis of these bob- 
bins, is shown in fig. 2, where 0 C . . . represent sections 
of the coils of wire by which they are filled. The scale of 
this section is half the real size of the object. At the centre 
of each of the bobbins a very small magnet, made of a piece 
of watch-spring about f of an inch long, and less than a grain 
in weight, is hung by a fibre of raw silk, the two magnets 
