GRAMMES MACHINE. 705 



1261. The preceding results are very approximately verified* for 

 machines, like those of Gramme, where the current is sensibly 

 uniform, either by measuring the electromotive forces, or by de- 

 termining the efficiency. Several causes however contribute to 

 complicate the effects. 



i st. The current is never strictly uniform, and the variations 

 in intensity require us to take into account the resulting extra 

 currents. 



2nd. It is probable that the soft iron in electromagnets is not 

 instantaneously magnetised, and this retardation in the magnetisation 

 gives rise to a loss of useful effect. 



3rd. For the magnets themselves, changes are produced in the 

 magnetisation in one direction or the other, and to unequal extents, 

 according as the actions are repulsive or attractive, which again 

 causes an analogous loss. The same is the case for electromagnets. 



4th. Magnets and cores of magnets are finally the seat of induced 

 currents, or what are called FoucauWs currents, which heat them to 

 the detriment of the usual work. 



The influence of all these reactions may be represented, at any 

 rate approximately, by adding to the circuit a fictive resistance which 

 is sensibly proportional to the velocity of the machine. 



1262. GRAMME'S MACHINE. Pacinottif devised the principle 

 of this machine as long ago as 1860, but his invention remained 

 unknown and without application. M. Gramme \ subsequently re- 

 discovered the same principle, and had the merit of applying it to 

 the construction of true industrial machines ; these machines have 

 served as models for most of those used at the present day. 



The principal part is a coil made up of a series of coils B 

 (Fig. 253) surrounding a ring A of soft iron; the coils are con- 

 nected separately to a series of conducting plates or strips insulated 

 from each other, and arranged on the surface of a cylinder which 

 acts as a commutator; each of the individual coils may be formed 

 of several turns. The circuit of the coils is thus closed on itself, 

 but the two halves are traversed by currents in opposite directions ; 

 two brushes C and C' press on two strips which are diametrically 

 opposite, and serve to transmit the current into an external circuit, 

 and form the poles of the machine. 



* MASCART and ANGOT. Journal de Physique, Vol. vn., pp. 79, 363. 1878. 

 t A. PACINOTTI. Nuovo Cimento, Vol. xix., p. 378. 1864. 

 I GRAMME. Comptes rendus, Vol. LXXIII., p. 175. 1871. 

 VOL. II. ZZ 



