278 
POPULAR SCIENCE REVIEW. 
of a Voltaic battery should discharge itself into the other end of 
the same battery — the discharge of either end_,or both ends^may 
be made into the earth ; and this is the course pursued in prac- 
tice — I mean in telegraph practice — for passing signals from 
place to place. A wire is led from one end of the battery, and, 
when in the act of transmitting a signal, is connected with gas 
or water pipes, or with any metals, that are in good electrical 
communication with the earth immediately on the spot ; and 
a wire is led from the other end along telegraph poles, through 
tunnels, or under the sea, for miles and miles, as the case may 
require, and is there in like manner placed in good communi- 
cation with the earth ; and so long as the communication is 
maintained perfect, and until the battery is exhausted, as above 
described, a current of electricity will pass along the wire. 
One battery cell is, however, not sufficient to produce a sensi- 
ble result at a distant place ; for this purpose a series of cells 
more or less numerous are arranged, the zinc of one being 
connected metallically with the graphite of the next. For the 
Train- Signalling Instruments to be described in the sequel, 
ten or a dozen cells are used for short distances ; the number 
being increased as the distance is greater. 
A telegraph wire, while transmitting a voltaic current, is 
active along its entire length ; and there are many ways of 
turning this activity to account. We are concerned with one 
property alone : it is the powmr possessed by such a current 
of converting soft iron into a magnet. The power of a single 
current is small ; but the same current in its course may be 
made to act many 
times on the iron 
until the desired 
effect is produced. 
For this purpose a 
quantity of cotton- 
covered copper 
wire is wound 
round the iron, 
and the current 
is made to pass 
from end to end of 
the coil of wire, 
its effect on the 
iron being mul- 
tiphedby the num- 
ber of convolu- 
tions. Such an instrument is termed an electro-magnet, 
and is employed in its simplest form for interpreting voltaic 
currents into audible or visible language. A couple of brass 
bobbins 3 in. by 4 in., as in fig. 2, are prepared ; and upon each 
