46 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



1074. It is highly important in using the spark as an indication, by its relative 

 brightness, of these effects, to bear in mind certain circumstances connected with its 

 production and appearance. An ordinary electric spark is understood to be the bright 

 appearance of electricity passing suddenly through an interval of air, or other badly 

 conducting matter. A voltaic spark is sometimes of the same nature, but, generally, 

 is due to the ignition and even combustion of a minute portion of a good conductor ; 

 and that is especially the case when the electromotor consists of but one or few pairs 

 of plates. This can be very well observed if either or both of the metallic surfaces 

 intended to touch be solid and pointed. The moment they come in contact the cur- 

 rent passes ; it heats, ignites, and even burns the touching points, and the appearance 

 is as if the spark passed on making contact, whereas it is only a case of ignition by 

 the current, contact being previously made, and is perfectly analogous to the ignition 

 of a fine platina wire connecting the extremities of a voltaic battery. 



1075. When mercury constitutes one or both of the surfaces used, the brightness of 

 the spark is greatly increased. But as this effect is due to the action on, and probable 

 combustion of, the metal, such sparks must only be compared with other sparks also 

 taken from mercurial surfaces, and not with such as may be taken, for instance, be- 

 tween surfaces of platina or gold, for then the appearances are far less bright, though 

 the same quantity of electricity be passed. It is not at all unlikely that the com- 

 monly occurring circumstance of combustion may affect even the duration of the 

 light ; and that sparks taken between mercury, copper, or other combustible bodies, 

 will continue for a period sensibly longer than those passing between platina or gold. 



1076. When the end of a short clean copper wire, attached to one plate of an 

 electromotor, is brought down carefully upon a surface of mercury connected with 

 the other plate, a spark, almost continuous, can be obtained. This I refer to a succes- 

 sion of effects of the following nature : first contact, — then ignition of the touching 

 points, — recession of the mercury from the mechanical results of the heat produced at 

 the place of contact, and the electro-magnetic condition of the parts at the moment*, 

 — breaking of the contact and the production of the peculiar intense effect dependent 

 thereon, — renewal of the contact by the returning surface of the undulating mercury, 

 — and then a repetition of the same series of effects, and that with such rapidity as to 

 present the appearance of a continued discharge. If a long wire or an electro-magnet 

 be used as the connecting conductor instead of a short wire, a similar appearance may 

 be produced by tapping the vessel containing the mercury and making it vibrate ; but 

 the sparks do not usually follow each other so rapidly as to produce an apparently 

 continuous spark, because of the time required when the long wire or electro-magnet 

 is used both for the full development of the current (1101. 1106.) and for its complete 

 cessation. 



1077. Returning to the phenomena in question, the first thought that arises in the 

 mmd is, that the electricity circulates with something like momentum or inertia in 



* Quarterly Journal of Science, vol. xii. p. 420. 



