DR. C. W. SIEMENS ON THE DYNAMO-ELECTRIC CURRENT. 
1073 
increase of all the effects, accompanied by a diminution in the resistance of the machine, 
is observed when a cross wire is placed so as to divert a great portion of the current 
from the electro-magnet.” 
Some of the constructors of dynamo machines, namely: Mr. Ladd in this country, 
and Mr. Brush in the United States of America, have taken advantage of this sugges¬ 
tion, the latter with the avowed object in view of obviating spontaneous changes of 
polarity in effecting electro-precipitation of metals, and without perhaps having realised 
all of the advantages of which this mode of action is capable ; others have refrained 
from doing so on account of difficulties resulting, as I shall endeavour to show, from 
an insufficient examination into some important physical conditions that require 
attention in order to realise economical results. 
An ordinary medium-sized Siemens-Alteneck dynamo-electrical machine has wound 
on its rotating helix insulated copper wire of 2’5 man. diameter in 24 sections, repre¬ 
senting a resistance of ’4014 S. UA The four electro-magnet coils connected seriatim 
are composed of copper wire of 5 - 5 m.m. diameter, presenting a total resistance of 
0-3065 S. U. 
If (as has frequently been done) the wires of this machine were to be connected as 
suggested in Sir Charles Wheatstone’s original paper, thus making the outer 
circuit not continuous with but parallel to the coil circuit, and if the outer circuit had 
a resistance of one unit, it would follow that the total resistance to the current 
set up by the rotation of the armature would be reduced from '4 + '3 + 1 •=1 , 7 to 
•3x1 
*4+-—— = 0'61 unit, causing a great increase of current, the major portion (in the 
proportion of 10 to 4) would flow through the electro-magnets, thus causing a great 
increase of heating effect. The resistance of the field magnet must therefore be greatly 
increased, but if it were attempted to increase that resistance simply by reducing the 
diameter of the wire, and increasing the number of convolutions until the same thickness 
of coil was obtained, the magnetic excitement and with it the electro-motive force of the 
current produced at a given velocity of rotation would suffer a material decrease. The 
current flowing through the helix coil would moreover have to divide itself, and in 
order to reach the same limit in the outer circuit its intensity in the helix coil would 
have to be increased, causing it to heat more readily than before. It was necessary, 
therefore, to raise the effect of the magnet current to the same level as before with 
as small a proportion of the helix current as possible, in order to leave a maximum 
proportion of the current for the outer circuit. In order to effect this, the magnet 
bars had to be increased in length, and placed further apart so as to provide room for 
* The resistance coils used in these experiments were graduated according to the mercury system intro¬ 
duced by Dr. Werner Siemens, and adopted by the Telegraphic Convention at Vienna in 1868. The B. A. 
unit was determined in 1874 by Kohlratjsch to be 1'0493 S. U., or combined with Lorenz’s value of the 
S. U. afterwards adopted, 09 7 9 7 X 10 9 C. G. S. units—as much as 2 per cent, below its ascribed theo¬ 
retical value. Later determinations by H. E. Weber (Phil. Mag., March, 1878) makes the S. U. to be 
equal to 0955 x 10 9 0. G. S. units, and thus the ohm to be 02 per cent, higher than its ascribed value; if 
this latter value is used, the numerical results must be correspondingly altered. 
MDCCCLXXX. 6 Y 
