ELECTRIC LIGHT. 



277 



ever, to avoid the mechanical complications incident 

 to apparatus for accomplishing this, M. Gramme de- 

 vised the apparatus outlined in Fig. 13. This is a 

 permanent horseshoe magnet, between the poles of 

 which, N S, is placed a ring of soft iron, around which 



FIG. 



is wound a coil of insulated wire. This ring is not 

 a permanent magnet, but when placed in the position 

 shown becomes so by induction from the permanent 

 magnet. The two poles S' N' will then be estab- 

 lished in the ring. If the ring be caused to revolve, 



FIG. 13. 



the poles will remain unaltered in space that is to 

 say, they will remain at N' S' ; and it follows that 

 every portion of the ring will alternately become a 

 north and a south pole. The consequence is, that 

 the poles may be regarded as constantly traveling 

 through the iron ring at the same rate as that at 

 which it revolves, but in an opposite direction ; and 

 the effect on the wire coiled on the ring is then pre- 

 cisely the same as though the magnet in Fig. 13 



revolved within the wire which was held at rest. It 

 is on this translation of polarity that the Gramme 

 machine depends for its action ; and, to go back to 

 our starting-point, its difference from other machines 

 may be summed up in the fact that, while in the latter 

 the magnet may be regarded as alternately entering 

 and being withdrawn from the coil, in the Gramme 

 machine the magnet is to all intents constantly pass- 

 ing entirely through the coil. In order to collect the 

 electricity produced, the insulating material is re- 

 moved from the wire in a narrow band round the 

 outside of the ring, and two rubbing collectors take 

 it up in the ordinary way. 



The construction of the rins: is shown in Fig, 

 14. It is composed of a group of soft- iron wires, A, 



FIG. 14. 



insulated coils. The radius pieces E are insulated 

 from each other by ribbons of silk or India-rubber. 

 The end of the wire terminating one coil and the 

 beginning of the wire of the next succeeding coil 

 are each attached to one radius piece by loops and 



FIG. 15. 



notches in the way shown. The tails of the radius 

 bars are all grouped together round the central axis, 

 and they are rubbed against by suitable collectors 

 which take up the electricity. 



The standard machine used for illuminating work- 

 shops and factories is represented in Figs. 15 and 

 Ifi, and consists of two vertical frames of cast iron, 

 united by four bars of soft iron, B B B B, which 

 serve as cores for the electro-magnets C C C C. The 

 axis is of steel, and revolves on long bearings, which 



FIG. 16. 



can bo effectively lubricated a point of importance, 

 as the speed is high, ranging from 700 to 1,350 revo- 

 lutions per minute. The central ring, instead of be- 

 ing covered with a single wire attached by equal 

 portions to a common collector, is covered with two 

 wires, wound on side by side and united with two 

 collectors. The poles of the electro-magnets H 11 

 are much developed, embracing seven eighths of 

 the circumference of the ring. Four wipers (bafait) 

 J J pick up the electricity. The electro-magnets 



