NETT ELECTRIC LIGHT IN WESTMINSTER CLOCK TOWER. 269 
follows that the curreiits will flow on without interruption as 
long as the rotation continues. The familiar class experi- 
ments, of which we have already spoken, go far to explain most 
of the phenomena of magneto-electricity ; it will not, then, be 
necessary to dwell further upon primary principles. However, as 
it may not appear obvious what cause is immediately concerned 
in the production of the Grramme currents, we shall mention 
an experiment made by M. Graugain, which will give a clear 
view of the subject. That distinguished physicist procured a 
ring of soft iron, and wound it with silk-covered copper wire 
in such a manner that the helix and its core might rotate 
either separately or simultaneously. This done, it was ob- 
served, 1st, that the strongest current was developed when the 
helix alone was displaced ; 2ndly, that the current was weaker 
when the helix and its core rotated together ; and 3rdly, that a 
very feeble current, inverse of the other two, was produced 
when the ring alone revolved. From this we gather that the 
currents owe their origin less to the displacement of the con- 
sequent poles than to the rotation of the helix. It would even 
appear as though the changes constantly taking place in the 
magnetic condition of the ring oppose the development of the 
currents.* 
The wire which covers the ring is not one continuous piece, 
but is wound in lengths of about 10 yards each. The ends of 
two contiguous lengths are brought out from the bobbin and 
joined to metallic sectors, which are connected to as many 
copper conductors, placed axially on the spindle of the machine. 
These are insulated from each other by layers of silk, and it is 
essential that they be sufficiently numerous to form a compact 
cylinder, and that the insulating material be very thin. The 
currents are collected at the neutral points on the axis by metallic 
brushes, consisting of silvered copper wires held together by 
adjusting screws. These accessories ensure good soft contact ; 
and when duly attended to, prevent the occurrence of sparks 
from the extra-current, which are always injurious to an ap- 
paratus. There are four of these brushes ; two to collect the 
current, which is made to circulate round the iron cylinders, 
and the other two to collect the useful current. 
The horizontal section (fig. 2) shows the number and dis- 
position of the coils. When these are set in motion, very 
weak currents are generated by the remanent magnetism in 
the soft iron. The current from the bobbin near the driving- 
wheel is transmitted round the cylinders, and thus augments 
their magnetisation. This new increment now induces a 
stronger current in the bobbin, which in its turn excites a 
* Compare Dr. Ferguson, “Electricity/’ Appendix, p. 2G3. 
