142 Mr. 0. Heaviside on the Extra Current. 



The actual potential is the sum of (17) and (20), and the actual 



current the sum of (18) and (19). When 3 is large the initial 



charge may be neglected altogether. Considering only the 

 potential and current due to the initial current, we find that 

 the current in the wire consists of a series of decreasing waves 

 in opposite directions, causing corresponding changes in the 

 potential of the wire. At the first moment after disconnexion 



the potential of the end Q becomes positive =Va /_ nearly, 



and the end P negative to an equal extent. Provided this elec- 

 tromotive force suddenly developed is not sufficiently great to 

 cause a spark, this state of things is rapidly reversed, the end P 

 becoming positive and the end Q negative, which is followed 

 by another reversal, and so on till the energy of the initial 

 current is all used up against the resistance of the wire. 



It is obvious that the simplicity (?) of the above formulas 

 must be considerably departed from in all practical cases that 

 occur, as in the above c and s are assumed to be the same for 

 every unit of length of the wire, which cannot be true, except 

 perhaps in a coiled submarine cable. But we may be sure 

 that, in virtue of that property of the electric current which 

 Professor Maxwell terms its " electromagnetic momentum," 

 whenever any sudden change of current or of charge takes 

 place in a circuit possessing an appreciable amount of self- 

 induction, the new state of equilibrium is arrived at through a 

 series of oscillations in the strength of the current which may 

 be noticeable under certain circumstances. It is naturally diffi- 

 cult to observe such oscillations with a galvanometer ; but some 

 telegraph-instruments show them very distinctly. For instance, 

 there is Wheatstone's " alphabetical indicator." The pointer 

 of this instrument is moved one letter forward round a dial by 

 every current passing through it, provided the currents are 

 alternately positive and negative. Now if an insulated straight 

 wire a few miles in length is suddenly raised to a high poten- 

 tial by means of a single current of very short duration from 

 a magneto-electric machine, and then immediately discharged 

 to earth through the coils of an " indicator," the pointer does 

 not merely move one step forward, as it would if the discharge 

 consisted of a single current, but several steps, indicating a 

 succession of reverse currents. The same thing occurs when 

 a condenser of small capacity is first charged to a high poten- 

 tial and then discharged through the instrument. Expressed 

 in popular language, what happens is as follows. The first 

 discharge-current is first retarded by the self-induction of the 



