.s7A' 11 7//././.1/ .S7A.]//-;.Y.S, /-.A'.S. 223 



nr t.nt'T rasing, the intervening span- being filled up with 

 pounded charcoal, (>, or other bad conductor of heat. A hole is 

 pierced through die bottom of the crucible for the admission of a 

 rod of inm, platinum, or dense carbon, c, such as is used in electric 

 illumination. The cover of the crucible is also pierced for the 

 reception of the negative electrode, by preference a cylinder of 

 (mpiv-^-.-d carbon, d, of comparatively large dimensions. At one 

 rnd of a beam supported at its centre is suspended the negative 

 electrode. f/,by means of a strip of copper, or other good conductor 

 <>t' electricity, the other end of the beam being attached to a hollow 

 cylinder of soft iron, c, free to move vertically within a solenoid 

 roil of wire, presenting a total resistance of about 50 units or ohms. 

 By means of a sliding-, weight, //, the preponderance of the beam 

 in the direction of the solenoid can be varied so as to balance 

 the magnetic force with which the hollow iron cylinder is drawn 

 into the coil. One end of the solenoid coil is connected with the 

 positive, and the other with the negative pole of the electric arc, 

 and, being a coil of high resistance, its attractive force on the 

 iron cylinder is proportional to the electro-motive force between 

 the two electrodes, or, in other words, to the electrical resistance 

 of the arc itself. 



The resistance of the arc was determined and fixed at will 

 within the limits of the source of power, by sliding the weight 

 upon the beam. If the resistance of the arc should increase from 

 any cause, the current passing through the solenoid would gain in 

 strength, and the magnetic force overcoming the counteracting 

 weight, would cause the negative electrode to descend deeper into 

 the crucible ; whereas, if the resistance of the arc should fall below 

 the desired limit, the weight would drive back the iron cylinder 

 within the coils, and the length of the arc would increase, until 

 the balance between the forces engaged had been re-established. 



Experiments with long solenoid coils have shown that the attrac- 

 tive force exerted upon the iron cylinder is subject only to slight 

 variation within a range of several inches, that is, within the limits 

 when the iron cylinder has just entered the coil, and when it has 

 advanced a little beyond the point of half immersion, which cir- 

 cumstance allows of a range of several inches of nearly uniform 

 action on the electric arc. The accompanying diagram, Fig. 2, 

 Plate 21, represents the attractive force of a solenoid coil of this 



