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POPULAR SCIENCE REVIEW. 
them. The instrument is large, cumbrous, and expensive. The 
permanent magnets are liable in course of time to weaken, and, 
indeed, the apparatus has hitherto only been utilized for the 
single beam of the lighthouse. 
The system has, however, been revived in a modified form 
quite recently by Mons. de Meritens. 
A great advance was made on this stage by Dr. Siemens, 
Sir C. Wheatstone, and Mr. Varley, in 1867. It was shown, 
that if an induction coil be made to revolve in front of a soft 
iron electro-magnet, instead of before a permanent magnet, as 
in the earlier machines, the small amount of residual magnetism 
always latent in the iron, especially if it has once been magne- 
tized, causes feeble currents to be induced in the coil ; if these 
currents, or a portion of them, be sent round the iron magnet, 
into the wire surrounding it, the magnetization is increased. 
This again produces a proportionate increase in the induced 
currents in the coil; thus by a series of successive mutual 
actions, intense magnetization and very powerful currents are 
produced. Machines constructed upon this principle of reaction 
have been termed “ dynamo-electric ” machines, as indicating 
that dynamical force is converted into electricity. For the older 
form, where permanent magnets are employed, the term “ mag- 
neto-electric ” has been retained ; their power being limited by 
the strength of magnetization of the permanent magnets made 
use of. This discovery, by adding considerably to the power of 
electric machines, led to a corresponding diminution in their 
bulk, and also in their cost. Almost, if not quite all subsequent 
machines are “ dynamo-electric ” in construction, and based on 
the principle of mutual reaction.* 
Those of this kind best known in this country are the 
Siemens and Gramme machines. 
In the former the armature or induction bobbin, on Dr. Sie- 
mens’ principle, has its enveloping wire wound lengthwise, and 
parallel to the axis of rotation ; it rotates between semi-circular 
prolongations of large flat electro-magnets placed on either side 
of it; their north pole being immediately above, the south 
pole immediately below the axis of the bobbin. It is commonly 
made of three sizes, the smallest costing £ 70 , consuming 2-horse 
power to drive it, and giving, with 850 revolutions per minute, 
an illuminating power of 1,200 candles. The illuminating 
power of the two larger machines is of 6,000 and 14,000 candles 
respectively. 
The Gramme machine is composed of an induction coil hav- 
ing a ring or cylinder of iron revolving between a pair of electro- 
magnets. The ring is wound with a number of transverse circles 
* From a paper “ On the Present State of Electric Lighting,” read before 
the Brit. Assoc, in Dublin by J. M. Shoolbred, B.A., &c. 
