1 64 SCIENTIFIC APPARATUS. 



ductors and the increased power needed to drive the machine pre- 

 vented any further increase of effect All the machines yet 

 referred to give not a continuous current but a rapid succession 

 of currents in opposite directions, the direction being reversed 

 twice during each rotation of the armature. In 1871 Gramme 

 produced a machine which gives, in the course of one revolution 

 of the armature, any number of successive currents of short dura- 

 tion all in the same direction. In this machine, a soft iron ring 

 rotates, about an axis perpendicular to its plane, between the poles 

 of a magnet, so that one of its diameters always coincides with the 

 line joining the poles. This ring is wrapped with a coil of insu- 

 lated copper wire, each turn of wire being in a plane containing 

 the axis of the ring, and the ends are joined together so as to 

 make a continuous circuit. At frequent equal intervals throughout 

 the coil, branch-wires are connected with it, whereby it can be 

 joined with other conductors. The action of the machine is, 

 in its main features, as follows. The direction of resultant 

 magnetic force between the poles of the magnet and outside the 

 ring is along the line joining the poles, but on entering the 

 substance of the ring, the direction of resultant force spreads out 

 into two semicircular branches, which reunite at the other end of 

 a diameter. Consequently, through a convolution of the coil 

 encircling the part of the ring which at any instant is nearest 

 to one of the poles, the magnetic force is nothing; whereas, 

 after a quarter of a turn of the ring, when the same con- 

 volution has come to be half way between the poles, nearly half 

 the total magnetic force of the magnet acts through it. After a 

 second quarter-turn of the ring, the force acting through the same 

 convolution is again nothing ; after a third quarter-turn it reaches 

 a second maximum of opposite sign to the first; after a complete 

 revolution it is again nothing. Consequently, in accordance with 

 the general principle of magneto-electric induction stated above 

 (p. 163), a succession of currents is produced in the coil as it re- 

 volves. The rotation of the soft iron takes no essential part in the 

 action, as, from symmetry, one part of it is equivalent to another 



