1067.  When a short wire is used, all these effects disappear.  If it be only two or three inches long, a spark can scarcely be perceived on breaking the junction.  If it be ten or twelve inches long and moderately thick, a small spark may be more easily obtained.  As the length is increased, the spark becomes proportionately brighter, until from extreme length the resistance offered by the metal as a conductor begins to interfere with the principal result.

1068.  The effect of elongation was well shown thus:  114 feet of copper wire, one-eighteenth of an inch in diameter, were extended on the floor and used as a conductor; it remained cold, but gave a bright spark on breaking contact.  Being crossed so that the two terminations were in contact near the extremities, it was again used as a conductor, only twelve inches now being included in the circuit:  the wire became very hot from the greater quantity of electricity passing through it, and yet the spark on breaking contact was scarcely visible.  The experiment was repeated with a wire one-ninth of an inch in diameter and thirty-six feet long with the same results.

1069.  That the effects, and also the action, in all these forms of the experiment are identical, is evident from the manner in which the former can be gradually raised from that produced by the shortest wire to that of the most powerful electro-magnet:  and this capability of examining what will happen by the most powerful apparatus, and then experimenting for the same results, or reasoning from them, with the weaker arrangements, is of great advantage in making out the true principles of the phenomena.

1070.  The action is evidently dependent upon the wire which serves as a conductor; for it varies as that wire varies in its length or arrangement.  The shortest wire may be considered as exhibiting the full effect of spark or shock which the electromotor can produce by its own direct power; all the additional force which the arrangements described can excite being due to some affection of the current, either permanent or momentary, in the wire itself.  That it is a momentary effect, produced only at the instant of breaking contact, will be fully proved (1089. 1100.).

1071.  No change takes place in the quantity or intensity of the current during the time the latter is continued, from the moment after contact is made, up to that previous to disunion, except what depends upon the increased obstruction offered to the passage of the electricity by a long wire as compared to a short wire.  To ascertain this point with regard to quantity, the helix i (1053.) and the galvanometer (1055.) were both made parts of the metallic circuit used to connect the plates of a small electromotor, and the deflection at the galvanometer was observed; then a soft iron core was put into the helix, and as soon as the momentary effect was over, and the needle had become stationary, it was again observed, and found to stand exactly at the same division as before.  Thus the quantity passing through the wire when the current was continued was the same either with or without the soft iron, although the peculiar effects occurring at the moment of disjunction were very different in degree under such variation of circumstances.