Analytical Study of the Alternating Current Arc. 317 



taken up in the carbons themselves, right up to the point 

 where the arc is being formed. The observations, when 

 reduced by applying these corrections, gave us at once the 

 true mean power being taken up in the arc. The wattmeter 

 was constructed with all the precautions necessary in making 

 a wattmeter for measuring alternating- current power, so that 

 it served to measure the power taken up either in alternating 

 or continuous current arcs. The current supplied to the 

 arc passed through a series of non-inductive resistances, con- 

 sisting of carbon plates, in such a manner that the power 

 given to the arc could be regulated with the greatest exact- 

 ness. In addition to the wattmeter, an ammeter was placed 

 in series with the arc lamp so as to measure the current 

 passing through the arc, and a voltmeter connected to the 

 carbons so as to measure the potential-difference of the 

 carbons, these instruments being suitable both for continuous 

 and alternating currents. In all the experiments the mean 

 power was kept constant in the arc, and this was done by 

 adjusting the current so that the wattmeter took a certain 

 deflexion corresponding to the power desired, and the watt- 

 meter was kept at a constant deflexion by regulating the 

 carbon resistances in series with the arc. In addition to the 

 instruments described above, a lens was fixed so as to enable 

 the length of the arc to be measured in the usual manner. 



In the course of the experiments three arc lamps were 

 employed — a hand-regulated arc lamp in which the distance 

 of the carbons was adjustable by a screw with great accuracy ; 

 a continuous -current arc lamp (the Waterhouse Arc Lamp); 

 and an alternating-current arc lamp (the Helios Arc Lamp) ; 

 these last two being selected as excellent arc lamps of their 

 respective types, our object being to select an arc lamp, the 

 mechanism of which enabled it to be worked with electric 

 powers varying over wide limits, and yet to yield a perfectly 

 steady arc. In addition to these instruments there was set 

 up the apparatus for delineating the curves of current of 

 electromotive force, which have been described by one of us 

 in the ' Electrician,' vol. xxxiv. p. 460, and which consists of 

 a synchronizing alternating-current motor, having its fields 

 separately excited and its armature circuit traversed by a 

 shunt current from the circuit operating the arc lamp. This 

 alternating-current motor was set up on the photometer bench, 

 and it was used to drive an aluminium disk pierced by four 

 openings, in such a manner that the disk revolved synchro- 

 nously with the alternating current operating the arc. In 

 addition to this duty the alternating-current motor carried a 



Phil. Mag. S. 5. Vol. 41. No. 251. April 1896. Z 



