48 



Prof. J. A. Fleming 



[Feb. 14, 



Fig. 16. 



with the electric arc. So strong is the current flowing in a circuit 

 connecting the middle carbon with the positive carbon that I can, as 

 you see, ring an electric bell and light a small incandescent lamp 

 when these electric- current detectors are placed in connection with 

 the positive and middle carbons. 



We also find that the flame-like projection of the arc between the 

 negative carbon possesses a unilateral conductivity. I join this small 

 secondary battery of fifteen cells in series with the galvanometer, and 

 connect the two between the middle carbon and the negative carbon 

 of the arc. Just as in the analogous experiment with the incandescent 

 lamp, we find we can send negative electricity along the flame of the 

 arc one way but not the other. The secondary battery causes the 

 galvanometer to indicate a current flowing through it when its 

 negative pole is in connection with the negative carbon of the arc 

 {see Fig. 16), but not when its positive pole is in connection with 



the negative carbon. On ex- 

 amining the third or middle 

 carbon after it has been em- 

 ployed in this way for some 

 time, we find that its extremity 

 is cratered out and converted 

 into grajjhite, just as if it had 

 been employed as the positive 

 carbon in forming an electric 

 arc. Time forbids me to indulge 

 in any but the briefest remarks 

 on these experiments; but one 

 suggestion may be made, and 

 that is that they seem to indicate 

 that the chief movement of car- 

 bon molecules in the electric arc 

 is from the negative to the posi- 

 tive carbon. The idea suggests 

 itself that, after all, the crater- 

 Galvanometer^ and battery B inserted - ^^^ ^^ ^^^ positive carbon of 

 in series between negative carbon ot , ® it, t 



electric arc and a third carbon to .how ^^^ arc may be due to a sand- 

 unihiteral conductivity of the arc be- blast-like action of this torrent 

 tween the negative and third carbons, of negatively-charged molecules 



which are projected from the 

 negative carbon. If we employ a soft iron rod as our lateral pole, we 

 find that, after enduring for some time the projection of the arc 

 against it, it is converted at the extremity into steel. 



Into the fuller discussion as to the molecular actions going on in 

 the arc, the source and nature of that which has been called the 

 counter-electromotive force of the arc, and the causes contributing to 

 produce unsteadiness and hissing in the arc, I fear that I shall not 

 be able to enter, but will content myself with the exhibition of one 

 last experiment, which will show you that a high vacuum, or, indeed, 



