354 RECENT PROGRESS IN PHYSICS- 



normal magnetization correspond in this case to the abnormal 

 periods. 



Hankel applied himself to the explanation of this phenomenon, 

 and he lays down the following as the fundamental idea : 



" It is known from Faraday's researches, that a current at its com- 

 mencement generates an opposite current in a neighboring conductor ; 

 at its cessation, on the other hand, a second current which passes in 

 the same direction with the original one. The electrical sparks must 

 act in both ways, upon a steel needle placed near the wires, as the 

 needle is perpendicular to the direction of the current, the planes of i 

 the currents produced in the needle are likewise perpendicular to the j 

 length of the needle, and the magnetism of the needle will be inii 

 opposite directions according as we consider it to be excited by the i 

 action of the beginning or by the cessation of the spark. But the two« 

 instants of beginning and end of electrical sparks follow each other sos 

 rapidly, that their separate? effects cannot be measured; -hence magnet-;) 

 ization is the result of both of these influences." j 



This is essentially the fundamental idea to which Wrede (SerzeZm' Ji 

 Jahreshericht, deutsch von Wohler, 20ster Jahrgang, S. 119,) soughtll 

 to reduce the alternate normal and abnormal magnetism of steel needles ' 

 by the discharge stroke in main as well as in secondary wires. 



As already intimated by Kiess, {Dove's Repertorium, VI, 218,) this 

 mode cf explanation belongs yet to the domain of conjecture. It is 

 possible that this is the natural process in magnetizing steel needles 

 loj the discharge stroke, but it is by no means proved. 



On the whole this explanation seems very plausible; but the deduc- 

 tion of the particulars of the phenomenon is not at all convincing, 

 although Hankel expresses himself quite at length upon the subject. 

 We will do well to consider this as still an open question. 



Riess remarks, in the place above cited in Dove's Repertorium, that 

 it is better, and more for the furtherance of science, openly to confess 

 the deficiencies of our knowledge, than to attempt to aid it with half 

 explanation and to cover up its defects ; and in this connexion he 

 quotes a passage from Franklin's letters, which should be taken to 

 heart by every scientific man : 



*^I find a frank acknowledgment of one's ignorance is not only the 

 easiest way to get rid of a difficulty, but the likeliest way to obtaini 

 information ; I think it an honest policy." 



In the second memoir Hankel treats of the following points : 



1. The number and magnitude of the magnetizing periods, men- 

 tioned in the first memoir. 



2. The action of different spirals. 



3. The action of the conducting wire upon itself. 



4. The influence of the thickness of the needles. 



5. The influence of the surface of the battery. 



6. The changes of the alternations by obstacles interposed. 



7. Special influence of particular metals^ totally distinct from theii 

 conducting capacity. 



We will consider these points in succession : 



1. As a magnetizing spiral, a spiral of silver wire was employee 

 with coils so close that the introduced needle covered 31 of them, 



I 



