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 current 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, between 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 commonly 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 succession 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[A], —­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.

  [A] Quarterly Journal of Science, vol. xii, p. 420.

1077.  Returning to the phenomena in question, the first thought that arises in the mind is, that the electricity circulates with something like momentum or inertia in the wire, and that thus a long wire produces effects at the instant the current is stopped, which a short wire cannot produce.  Such an explanation is, however, at once set aside by the fact, that the same length of wire produces the effects in very different degrees, according as it is simply extended, or made into a helix, or forms the circuit of an electro-magnet (1069.).  The experiments to be adduced (1089.) will still more strikingly show that the idea of momentum cannot apply.