174 SECTION PHYSICS. 



variable state of currents by an exponential curve, differing from that 

 of Ohm, and without a point of inflexion; M. Rijke, who, by an 

 ingenious, but, as he himself confesses, not sufficiently accurate pro- 

 cess, tried to determine the duration of extra currents ; M. Guillemin, 

 who confirmed Ohm's theory with regard to long telegraphic lines; 

 and finally Sir W. Thomson, who arrived at far different conclusions 

 from the others. 



This was the state of the question when I began to interest myself 

 in it. After me, it occupied the attention of many scientific men, 

 among them MM. Helmholtz, Lemstrom, Bernstein, Felici, Donati, 

 Cazin, and others. 



A certain number of instruments have been made, of which I shall 

 mention the most important : The apparatus of M. Helmholtz upon a 

 system of levers ; it has also been employed by M. Lemstrom. My 

 apparatus, with a revolving cylinder, and with metallic contacts, after 

 the idea of MM. Wartmann and Dove. The second apparatus of 

 M. Helmholtz, in the form of a pendulum, with metallic contacts. The 

 apparatus of M. Bernstein, revolving, with contacts of mercury. The 

 apparatus of M. Cazin, a weight, which in falling forms a current. 

 Finally, the apparatus of M. Felici, revolving, with an inscribing 

 diapason for the measurement of time. 



The principal fact which resulted from my researches, and which has 

 been confirmed and amplified by other scientific men, is that the 

 variable state of currents is occasioned by a phenomenon far more 

 complex than it was supposed to be. The intensity of the current, 

 which is at first zero, reaches a degree nearly double that of its normal 

 value ; then descends nearly as far as zero but without quite reaching 

 it ; then attains a second maximum, rather less than the first ; then 

 descends to a second minimum, less marked than the first. Thus the 

 current makes a series of fluctuations around its final value. The 

 duration of an oscillation does not appear to be always the same. The 

 conditions, on which it depends, are not yet known ; but it seems that 

 it can vary from a half to four ten-thousandth part of a second. The 

 number of these oscillations is very great. I have found traces of them 

 to the hundredth parts of a second. The number which can be 

 observed depends evidently upon the delicacy of the galvanometer used 

 It depends also upon the shape of. the circuit. When in the circuit 



