DISRUPTIVE DISCHARGE — GLOW — CHANGED INTO BRUSH OR SPARK. 137 



difficult to convert the glow into brushes, by affecting the current of air (1574. 15/9.) 

 or the inductive action near it. 



1539. The transition from glow, on the one hand, to brush and spark, on the other, 

 and, therefore, their connexion, may be established in various ways. Those circum- 

 stances which tend to facilitate the charge of the air by the excited conductor, and 

 also those which tend to keep the tension at the same degree notwithstanding the dis- 

 charge, assist in producing the glow ; whereas those which tend to resist the charge 

 of the air or other dielectric, and those which favour the accumulation of electric 

 force prior to discharge, which, sinking by that act, has to be exalted before the 

 tension can again acquire the requisite degree, favour intermitting discharge, and, 

 therefore, the production of brush or spark. Thus, rarefaction of the air, the removal 

 of large conducting surfaces from the neighbourhood of the glowing termination, the 

 presentation of a sharp point towards it, help to sustain or produce the glow : but 

 the condensation of the air, the presentation of the hand or other large surface, the 

 gradual approximation of a discharging ball, tend to convert the glow into brush or 

 even spark. All these circumstances may be traced and reduced, in a manner easily 

 comprehensible, to their relative power of assisting to produce, either a continuous 

 discharge to the air, which gives the glow ; or an interrupted one, which produces the 

 brush, and, in a more exalted condition, the spark. 



1540. The rounded end of a brass rod, 0*3 of an inch in diameter, was covered 

 with a positive glow by the working of an electrical machine : on stopping the ma- 

 chine, so that the charge of the connected conductor should fall, the glow changed 

 for a moment into brushes just before the discharge ceased altogether, illustrating 

 the necessity for a certain high continuous charge, for a certain sized termination. 

 Working the machine so that the intensity should be just low enough to give conti- 

 nual brushes from the end in free air, the approach of a fine point changed these 

 brushes into a glow. Working the machine so that the termination presented a con- 

 tinual glow in free air, the gradual approach of the hand caused the glow to contract 

 at the very end of the wire, then to throw out a luminous point, which, becoming a 

 foot stalk (1426.), finally produced brushes with large ramifications. 



1541. Greasing the end of a rounded wire will immediately make it produce 

 brushes instead of glow. A ball having a blunt point which can be made to project 

 more or less beyond its surface, at pleasure, can be made to produce every gradation 

 from glow, through brush, to spark. 



1542. It is also very interesting and instructive to trace the transition from spark 

 to glow, through the intermediate condition of stream, between ends in a vessel con- 

 taining air more or less rarefied ; but I fear to be prolix. 



1543. All the effects show, that the glow is in its nature exactly the same as the 

 luminous part of a brush or ramification, namely a charging of air ; the only differ- 

 ence being, that the glow has a continuous appearance from the constant renewal of 



MDCCCXXXVni. T 



