296 POGGENDORFF ON THE PHiENOMENA OF 



had been proportional to the strength of the current, and the 

 mutual attraction of both proportional to the square of the 

 magnetic force, then by an augmentation of the current from 

 1 to 10 the lifting power should increase in the ratio of 1 : 100, 

 whereas in reality the increase is only in the ratio of 1 : 3! 

 This extraordinary divergence from the simplest assumption, 

 which certainly in no former experiments exhibited itself so 

 strikingly, is evidently to be referred to the reaction of the 

 keeper upon its magnet, by which the latter is enabled to as- 

 sume a higher degree of magnetism than could be imparted to 

 it by the action of the galvanic current alone. 



A glance at the progression of the lifting powers shows that 

 the addition to the magnetic power is greater in the case of the 

 weaker than of the stronger currents, or that the increase di- 

 minishes as the force already possessed by the magnet becomes 

 greater ; but we do not learn therefrom in what ratio the total 

 force of the magnet increases with the strength of the current, 

 and just as little, according to what law the magnet and keeper 

 attract each other. 



To obtain information on this head, the author resorted to 

 the method of measurement furnished by the induced currents 

 excited by an electro-magnet, which currents may be regarded 

 as proportional to the total amount of the developed magnetism. 

 He surrounded a horse-shoe magnet with two spirals, each of 

 which consisted of two well-insulated wires, and connected these 

 in such a manner that two circuits were formed, in one of which 

 the battery and the sine-compass were included, and in the other 

 the galvanometer used to measure the induced current. To 

 meet all the questions which suggest themselves here, the in- 

 duced currents were measured in four cases, namely, a with an 

 open magnet, b with a closed magnet and the first closing of 

 the primary circuit, c with a closed magnet and a second closing 

 of the circuit, and d through the puUing away of the keeper ; 

 which latter, in one series of experiments, was wide, and in the 

 other narrow, not being quite sufficient to cover the magnet. 



The following table will serve as an example of the results 

 obtained in this manner; the strength of the induced currents 

 is denoted by the letters a, b, c, d, the said strength being ex- 

 pressed by the sine of half the angle through which the needle 



