60 
NATURE 
[WVov. 16, 1882 
having poles set alternately all round a circular frame. 
Figs. 4 and 5 show how this is carried out. A cast-iron 
ring having projecting iron pieces screwed into it is sur- 
rounded by zig-zag conductors which carry into it the 
current from a separate exciter. These currents pass up 
and down between the projecting cheeks, and excite 
those on both sides of them. 
A still more recent, and still larger generator, is that de- 
signed by Mr. J. E. H. Gordon, whose “ Physical Treatise 
on Electricity and Magnetism”’ is known to most of our 
readers. This machine, which is given in elevation in Fig. 6, 
and in end-elevation in Fig. 7, is more than 9 feet in height, 
and weighs 18 tons. It possesses several points of interest. 
The rotating armature differs from those of the well-known 
Gramme or Siemens’ armatures, being in form a dsc, 
constructed of boiler-plate, upon which the coils are 
carried. The machine, therefore, resembles in some 
respects the Siemens’ alternate-current machine, though 
there are notable points of difference, the most important 
—— 
Gordon’s Dynamo. 
being, that whereas in most dynamo-machines the in- 
ducing field-magnets are fixed, and the induced coils 
rotating, in Mr. Gordon’s new machine the rotating coils 
are those which act inductively upon the fixed coils 
between which they revolve. The machine furnishes 
alternate currents, and therefore requires separate exciters. 
These exciters, two Biirgin machines, send currents which 
enter and leave the revolving armature by brushes press- 
ing upon rings of phosphor bronze placed upon the axis 
at either side. There are 64 coils upon the rotating 
disc, and double that number upon the fixed frame- 
work. These 128 “taking-off” coils, the form of which 
is shown in Fig. 8, are alternately connected to two 
circuits, there being 32 groups in parallel arc, each 
parallel containing 4 coils in series; thus bringing the 
total electromotive force to 105 volts when the machine is 
driven at 140 revolutions per minute. At this speed it 
actuates 1300 Swan lamps, but is calculated to actuate | 
from 5000 to 7000 if the driving power is proportionately 
increased, The machine is now in operation at the 
Telegraph Construction and Maintenance Company’s 
Works, East Greenwich. 
A great deal has been said in certain quarters of late 
about another new dynamo, the invention of Mr. 
Ferranti, which, with one of those unscientific exag- 
gerations which cannot be too strongly condemned, was 
pronounced to have an efficiency five times as great as 
that of existing dynamos. The construction of this ma- 
chine has not yet been made known, but it is understood 
that it has no iron in the rotating armature. This is, 
however, no novelty in dynamos. It appears, also, that 
Mr. Ferranti has invented an alternate-current machine 
almost identical with that of Sir William Thomson 
described above. 
Lastly, M. Gravier claims to have designed a form of 
dynamo in which there are neither commutators nor 
separate exciters, but in which continuous currents of 
electricity are produced in stationary coils by the passage 
near them of a rotating series of iron bars whose mag- 
Fic. 8.—The Fixed Coils of Gordon’s Dynamo. 
netism is changed, during their passage, by the reaction 
of the cores of the stationary coils themselves. M. Gravier 
has also designed a machine in which a Gramme-ring is 
wound with two sets of coils, a primary and a secondary, 
each set having its own commutator on opposite ends of 
the axis. A current from a separate exciting machine 
passes into the primary coils of the ring by one pair of 
brushes, and the secondary current is taken off by a 
second pair of brushes at the other commutator placed at 
right angles to the first pair. We are not aware that any 
practical machine thus constructed has yet been shown 
in action. 
It is certain that there is yet abundant room for great 
improvement in the construction of dynamo-electric 
machines. But the inducements to improvement at the 
present time are so great that rapid progress toward the 
desired goal of perfect efficiency and simplicity of structure 
is more than assured. 
THE PROJECTION PRAXINOSCOPE 
GASTON TISSANDIER describes in La Nature 
* an ingenious adaptation of the praxinoscope, under 
the above name, by means of which the images are pro- 
jected on a screen, and are visible to a large assembly. 
