.SVA' //'//./././. I/ SIEMENS, F.R.S. 



2I 7 



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 

 deofcro-magnet." 



S< une of the constructors of dynamo machines, namely : Mr. Ladd 

 in this country, and Mr. Brush in the United States of America, 

 li;i\v taken advantage of this suggestion, the latter with the 

 a vo\\ ed 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 condi- 

 tions 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 m.m. diameter in 24 sections, representing a resistance of 

 4014 S. U.* The four electro-magnet coils connected seriatim 

 are composed of copper wire of 5'5 m.m. diameter, representing 

 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 resis- 

 tance 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- = T7 to '4 + 



3x1 

 1+-3 



= 0'G1 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 



* The resistance coils used in these experiments were graduated according to 

 the mercury system introduced by Dr. Werner Siemens, and adopted by the 

 Telegraphic Convention at Vienna in 1868. The 15. A. unit was determined in 

 1874 by Kohlrausch to be 1'0493 S. U., or combined with Lorenz's value of the 

 S. U. afterwards adopted, 0'9797 x 10 9 C. G. S. units as much as 2 per cent, 

 below its ascribed theoretical value. Later determinations by H. F. Weber (Phil. 

 Mag., March, 1878) makes the S. U. to be equal to 0'955 x 10 9 C. G. S. units, 

 and thus the Ohm to be '2 per cent, higher than its ascribed value ; if this latter 

 value is used, the numerical results must be correspondingly altered. 



