Correspondence. ltt 
Let us consider for a moment figure 3. When the current is estab- 
lished, the electro-magnet is divided into two parts magnetically dis- 
tinct, one boreal, m, and the other austral, n’, and since the helix sur- 
Figure 4 represents a pulley with two channels, and consequently, 
with two helices, by means of which we may produce an electro-mag- 
net “a point conséquent” if a contrary direction is given to the current, 
or if a normal electro-magnet having contrary poles at the extremities 
is used and ihe currents have the same direction 
This magnetic traction, operating as seen between turned pullies 
which are consequently without teeth, is attended with much less fric- 
tion than where there are ordinary cog wheels; and the rapidity of the 
minute, he sum of the thicknesses of the three circles was seven 
centimeters. In the next number, | will treat fully of this subject of 
fact that at each induction, the inducing wire gives e to an in- 
duced current which increases considerably the electric tension at the 
point where a spark is given from the interruptor. is spark acts 
itis true on connections of platinum: but its energy is such that the 
surfaces are soon tarnished, fused and bored through; and the vibra- 
tions becoming thus less constant, the production of electricity ceases 
to be regular. An attentive study of this machine ed } u 
. Several arrangements satisfactorily prove this, such as the 
use of metals, less liable to be acted upon than platinum, upon the 
surfaces of the interruptor, and uniting the vibrating parts by fine wires 
of different lengths. : 
he means of increasing the power of the machine is then to pre- 
vent the development of the current which is produced in the inducing 
wire at the moment of the rupture of the circuit, by acting directly on 
feeble. M. Fizeau 
. 
the tension of the current and rendering it more . 
