6 Prof. J. C. Posrgendorff on the Extra Current 



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introduced into the path of the current. The striking-distance 

 stands obviously in a direct ratio to the electromotive force ; and 

 when this latter is enfeebled by an opposite current, it can scarcely 

 be otherwise than that the striking-distance should also be 

 diminished. Although my earlier investigations seem to speak 

 against this, I believe that such a diminution actually occurs 

 whenever the interposed wire is used in the form of coil, and 

 that it is only on the one hand the indefiniteness of the striking- 

 distance, and on the other the weakness of the partial current, 

 which may have prevented this diminution from being perceived. 

 It does not here depend so much on the absolute length of the 

 coil added, as upon its ratio to the length of the wire of the in- 

 ductorium. With a certain ratio, the opposite current, or its en- 

 feebling influence upon the inducing current, is most strongly 

 developed, and then the diminution also naturally follows. 

 Some experiments which I made in this respect were favourable 

 to this view, although a repetition of them with greater means 

 than those at my command would not have been superfluous. 

 For straight wires, the above statement, though not perhaps with 

 the utmost rigidity, applies with tolerable approximation. 



In the foregoing I have only spoken of the developments of 

 the opposite current in free air; its occurrence is extremely 

 striking when part of the circuit is in a rarefied space. 



If under the receiver of an extra plate of an air-pump which 

 is provided with the necessary insulated conductors, a bright 

 copper wire is stretched, and the air is adequately exhausted, 

 and if the arrangement is placed in the circuit of an inducto- 

 rium provided with its accessory coil, as soon as the instrument 

 is set to work the wire is seen to become brightly luminous, and 

 to send bright rays towards the bell. The phenomenon is im- 

 proved by clothing the bell externally with a strip of tinfoil 

 corresponding to the wire, which is placed in connexion with 

 the ground ; and still more by bringing a small piece of phos- 

 phorus under the bell. 



In general the wire does not become continuously, but partially 

 luminous ; these luminous parts are in continual motion, run 

 backwards and forwards on the wire, and send glimmering rays 

 towards the tinfoil, which also becomes luminous on the inside, 

 so that the whole, since at the same time the dark parts, by- 

 contrast, appear to emit dark rays, has an appearance like that 

 of the aurora borealis. 



I could see nothing of a stratification in this luminous phe- 

 nomenon, although I had first with this view allowed phospho- 

 rus to evaporate under the bell ; the formation of stratification 

 is probably suppressed or concealed by the great mobility of the 

 light. Nor could I perceive any material difference in the appear- 



