on the Alternate Current Arc. 509 



•which was a " Wilde " alternator, the armature of which 

 contained iron. 



It will also be noticed that the first ordinates of the curves 

 are not quite equal to the last ordinates ; this is due to the 

 curve being slightly asymmetrical. The whole period is only 

 one third of the revolution of the alternator (shown as 120 

 degrees in the figure). In one complete revolution this lack 

 of symmetry would of course disappear. 



It is seen that the arc alters the wave form from a peaked 

 curve to a very flat topped curve, changing sign almost 

 instantaneously, and with two small maxima, which occur 

 respectively just before and just after each reversal. The 

 next curve shows the effect of adding a resistance of 1 ohm 

 in series with the arc. This smooths out the two maxima, 

 but otherwise does not affect the shape materially. 



The current and P.D. curves with 5 ohms in series with 

 the arc are next shown. Both waves now assume much more 

 the form of the E.M.F. curve on open circuit, except at the 

 zero line. Here the P.D. curve crosses the line twice in each 

 direction, and the current curve runs parallel to the zero line 

 for some way before crossing it. 



These curves show, I think, the great effect that the arc has 

 on the wave form, and also how this effect can be destroyed 

 by placing resistance in series with the arc. 



In a paper by Rossler and Wedding, which appeared in the 

 'Electrician' for August 3 1st, 1891=, it was proved that an 

 alternate current arc is more efficient, that is, gives a higher 

 candle-power for the same electric power consumed, when the 

 alternating current feeding it has a flat-topped than when it 

 has a peaked wave form. 



Rossler was, however, mistaken, I think, in assuming that 

 the machine was making that curve. Indeed, this mistake 

 runs through the whole of this otherwise most valuable paper. 

 Rossler took three machines giving, as he thought, wave forms 

 from the extremely peaked to the extremely flat wave, and 

 determined the efficiencies of the same arc lamp for each of 

 them. Whereas exactly the same results might have been 

 obtained from one machine alone on causing it to give a 

 higher voltage by increasing the field excitation and then 

 absorbing the excess of voltage in resistance, exactly as in 

 the case of the Wilde alternator above referred to. 



I understand that in the experiments described by Dr. 

 Fleming and Mr. Petavel there was always a resistance 

 amounting to 7 ohms outside the arc, and hence a wave form 

 was forced upon the arc which, as Rossler has proved, is not 

 the most efficient one, and which the arc would convert into a 



