1072 
DR. C. W. SIEMENS ON THE DYNAMO-ELECTRIC CURRENT. 
with the commutator, and through it, with the wire surrounding the soft iron bars, 
and with the electric lamp or other resistance on the outer circuit. 
The advantage claimed for this mode of construction is that all the wire forming 
the rotating coil or helix is brought into the magnetic field, excepting only those 
portions crossing from side to side of the coil; and in order to reduce this unproductive 
resistance to a minimum, the rotating coil or helix has been made comparatively long, 
and the number of electro-magnets has been increased generally to six or more. 
The principal advantage of the dynamo-electrical machine over all other current 
generators consists in its power of producing currents of great magnitude, and of an 
intensity up to 100 volts, with a small primary resistance, and therefore with a com¬ 
paratively small expenditure of mechanical energy. It labours, on the other hand, 
under the disadvantage that the power of the current depends, at a given velocity, 
upon the magnetic force developed in the electro-magnets. This force depends upon 
the amount of current passing through the coils of the magnets, which in its turn is 
dependent in an inverse ratio upon the resistance in the outer circuit. If from some 
accidental cause the external resistance is increased, the electro-motive force of the 
machine, instead of rising to overcome the obstruction, diminishes, and thus aggravates 
the resulting disturbance. If, on the other band, the resistance of the outer circuit 
diminishes, as in the case when the carbons of an electric regulator touch one another, 
the electro-magnets are immediately excited to a maximum, and the electro-motive 
force of the machine is increased. The power absorbed and its equivalent, the heat 
generated in the circuit, is equal to the square of the electro-motive force divided by 
the resistance; hence the work demanded from the engine will be greatly increased, 
the machine may be dangerously overheated, and powerful sparks may injure the 
commutator. It is chiefly owing to this instability of the dynamo-electric current 
that its application to electric illumination has been retarded, and that magneto¬ 
electric machines and machines producing alternating currents have been again used, 
although they are inferior to the dynamo machine in the current energy produced for 
a given expenditure of mechanical energy. 
The properties of dynamo-electric machines have been examined by several 
observers. Messrs. Houston and Thomson (Franklin Institute) compared the 
efficiency of the Gramme, Brush, and Wallace Farmer machines. Dr. Hopkinson 
(Institution of Mechanical Engineers, 25th April, 1879) examined a medium-sized 
Siemens machine, determined its efficiency, and expressed the electro-motive force as 
a function of the current. Herrn Mayer and Anerbach (Wiedemann’s ‘ Annalen,’ 
November, 1879) experimented on a Gramme machine, and obtained a curve very 
similar to Diagram I. M. M a sc a rt has experimented on the Gramme machine, and 
Mr. Schwendler on both Gramme and Siemens machines. 
The radical defect of the dynamo machine of ordinary construction, may be inferred 
from the results of these experiments. The remedy has, however, been in our hands 
from the time of the first announcement of the principle of these machines before the 
Royal Society, when Sir Charles Wheatstone pointed out that “ a very remarkable 
