148 ELECTRIC LIGHTING. 



current is proportional to the speed, the resistance of the coil 

 being constant. 



" We thus reach a limit where the distance between the elec- 

 trodes becomes infinitely small, that is to say when the electrodes 

 are in contact. This is when their sections are nearly as i to 

 64 ; then the negative electrode scarcely gets heated, and it is 

 therefore not consumed. Under these conditions it is only the 

 positive electrode which is consumed, while it produces a beauti- 

 ful absolutely fixed white light so long as the intimate contact 

 between it and the negative electrode is maintained. It is in 

 reality a light produced by an infinitely small voltaic arc. 



When the reverse method is adopted, that is to say, when, 

 instead of lessening the section of the positive electrode, the 

 section of the negative is gradually reduced, and the section of 

 the positive increased, the light of the latter is seen to gradually 

 diminish, and the heat of the negative electrode to increase. 



"When the sections of the electrodes are nearly as i to 64, 

 and they have been placed in contact, no light is any longer 

 given off by the positive electrode ; the negative one alone pro- 

 duces the light. It is curious that when a voltaic arc is set up 

 between the two carbons, the smaller electrode is always shaped 

 into a point, whether it be positive or negative." 



Fig. 37 represents the series of changes in the form of the 

 electrodes when their respective dimensions are made to 

 vary. The electrodes in the centre represent the ordinary 

 electrodes of equal section. In the three systems on the 

 left are shown the effects produced as the lower electrode, 

 which is the positive one, increases, and the three on the 

 right show the effects of a successive increase of the upper 

 and negative electrode. 



As we have seen, Werdermann has been able by derivation 

 to obtain with an electroplating Gramme machine the light- 

 ing of 10 electric lamps. The resistance of the coil of the 

 machine was o'ooS ohm, and the electro-motive force was 

 equal to that of 4 Daniell cells, with a speed of 800 turns 

 per minute. At this speed the current corresponded with 

 66*06 webers; but with a speed of 900 turns it corresponded 



