DR. C. W. SIEMENS ON THE DYNAMO-ELECTRIC CURRENT. 
1075 
Appendix. 
The measurements of the electric currents were made with an electro-dynamometer, 
the movable part of which consisted of a single turn of 4 m.m. wire, and the stationary 
coil of nine turns of the same. 
To be able to reduce the electrical measurements into absolute power developed, it 
was in the first place necessary to determine the constant of the instrument in use. 
This was done in the following manner:—Five copper plates of about 11" X 8" were 
connected as shown in the sketch. 
These were carefully weighed and immersed in a solution of sulphate of copper. 
The machine was previously started, the time of immersion carefully noted, and the 
readings of the current taken every half minute. The plates were so arranged that 
the current entering at a and leaving at 2 deposited the copper on both sides of the 
plate at 2 . After a certain time the plates were taken out, quickly rinsed in water, 
and dried in sawdust. The plates were then carefully weighed again and the deposit 
calculated per degree reading on the instrument per second of time. Six independent 
measurements were taken with currents varying from 20 to 40 webers, and gave a 
mean of '000779 gramme of copper per second per degree reading. The differences 
of these measurements from the mean varied from 0'21 per cent, to 6'6 per cent., the 
mean of the differences being 1*98 per cent. 
According to F. Kohlrausch (Pogg. Ann., Bd. cxlix., 1873) the quantity of silver 
deposited by the C. G. S. unit of electricity is 0 011363 gramme, and since the 
quantities vary as the equivalents of the metals deposited, we have 
•011363x63-5 „ „ 
—-= 0 - 003340 gramme ol copper. 
One weber being C. G. S. unit, we have to divide by 10 the quantity of copper 
deposited by a current of one weber in one second, that is '000334 gramme, and dividing 
'000779 by '000334 we get 2'23323 webers for a degree reading of our instrument. 
To be able to compare the machines having the new winding (i.e., the wire on the 
electro-magnets connected parallel with the outer circuit) with the ordinary machines, it 
was necessary to experiment on the relation existing between the power expended and 
the current produced with different resistances in circuit and different speeds. 
A medium dynamo machine with 24 part commutator was used, the helix being 
wound with 336 convolutions of 2'5 m.m. wire, having a resistance of '4014 S. U. 
when measured in the machine. The electro-magnets were wound with four layers of 
6 y 2 
