DOVE ON THE ELECTRICITY OF INDUCTION. 161 



physiological action of a spiral by the insertion of solid iron ; 

 although in cases where higher orders can be examined, it might 

 perhaps be attained with a greater number of spirals coiled for 

 this particular purpose. As it is certain that the curi'ents in- 

 duced by electro-magnetized bundles of wires, from the proper- 

 ties which they have been shown to possess generally, fill up by 

 a number of intermediate grades the wide gap between conti- 

 nuous galvanic currents and the momentary currents of frictional 

 electricity, so in all probability the secondary currents of higher 

 orders will be the means of supplying the remaining omissions 

 between those two extreme members of the series. 



The researches which have been detailed in this memoir have 

 tended to point out the influence w^hich is exerted by the pre- 

 sence of solid iron and insulated bundles of iron wires upon 

 induced currents excited by primary curi'ents from different 

 sources, both when they were developed as adjacent currents 

 in separate wires, or existed in the form of the so-called extra 

 currents in the coils themselves of the connecting wire, on 

 disconnecting it with the source of electricity. It now remains 

 to be examined what influence this iron exerts on the initial 

 counter current which a commencing primary current produces 

 in its own coils. As however nothing whatever is known of 

 the physiological action of this current, the first requisite w^as 

 to invent means to produce it in such a manner as would admit 

 of rheometric measurements being applied to it. The follow- 

 ing sections contain the account of the researches undertaken for 

 this purpose. 



VIII. Extra current at the commencement and close of a primary 

 current, and its modifications by the presence of iron. 



As an electric cuirent, the intensity of which is increasing, 

 may be considered at any moment as consisting of tw^o portions, 

 the one of which the constant part remains unchanged, the 

 other is that which is constantly being added; and again in a 

 current the intensity of which is decreasing, the portion which is 

 leaving it may be distinguished from the constant portion, then 

 the law of induction, that a primary current induces at its com- 

 mencement a current flowing in an opposite direction, and at its 

 cessation, one in a like direction, that it induces no current at 



