266 
rOPULAR SCIENCE EEYIEW. 
Accordingly, we find many labourers in the field even from an 
early hour. Some centred their attention upon the inductive 
action of voltaic currents, and among the most successful of 
these as candidates for fame and fortune was Euhmkorff. Others 
devoted their thoughts to the magneto-electric spark, and de- 
vised apparatuses, which, in their days, startled the scientific 
world. But such is the discarding nature of progress that the 
beautiful contrivances of Pixii, Saxton, and Clarke are now 
objects more of curiosity than utility, and are therefore relegated 
to collections of physical instruments. These expensive toys, 
as they were called, have been succeeded by machines which 
produce veritable streams of electricity. These changes have 
been effected rapidly indeed, but not per saltum ; and it is as 
instructive as entertaining to follow up the steps which have 
led to these results. 
The rationale of voltaic induction, as determined by th6 
labours of Faraday, is that a current which begins or finishes 
generates in a neighbouring conductor currents whose directions 
may in every case be determined by Lenz’s law. The transition 
from this to magneto-electric induction was comparatively easy, 
as Ampere had many years before propounded to the Academy 
of Sciences his famous electro-dynamic theory, which virtually 
identified magnetism and electricity. Experiment proved the 
accuracy of the Amperian theory, and Faraday was the first to 
elicit the spark from the magnet. Every tyro in science knows 
that the introduction of a magnet within a hollow coil produces 
an inverse current in the wire, as indicated by the oscillations 
of the galvanometer needle. These effects may be rendered 
much more striking by inserting within the coil a bar or a 
cylinder of soft iron, and then approaching and withdrawing a 
pole of a magnet. The pole acts inductively on the soft iron ; 
the variation of distance augments or decreases its magnetic 
condition, and this fluctuation suffices to generate the induced 
current. The strength and continuance of the latter depend 
on the velocity and duration of the movement. It may be 
interesting to vary these experiments by winding a few turns 
of copper wire — the free ends of wliich are connected with a 
galvanometer — on a soft-iron bar, whose middle section is at 
right angles to a steel magnet, and then moving the helix from 
the centre to either extremity and back again, when it will be 
noticed that in the first case the currents are in the same direc- 
tion, and if we call them direct, those in the second will be 
inverse. 
Gauss and Weber were the first to utilise Faraday’s disco- 
very ; they transmitted telegraphic signals by raising or depress- 
ing a wooden bobbin, wound with copper wire, through the 
centre of which passed two or three vertically placed bar mag- 
