INVESTIGATION OF CURRENTS IN THE VARIABLE STATE. 297 



capacity of the condenser added to that of the wire forms the 

 total capacity C. 



The apparatus thus charged by induction is then left to itself. 

 The discharge through the induced wire is continuous or oscil- 

 lating (536), according as we have one or other of the conditions 



Von Helmholtz used in his experiments a small Ruhmkorff's 

 coil without a soft iron core. In order to determine the condition 

 of the induced wire at the time /, after opening the inducing current, 

 the sciatic nerve of a frog, which is still the most sensitive galvano- 

 scope, was introduced at this moment by means of a key. 



The intensity of a discharge current is a maximum when the 

 charge of the condenser passes through zero (538) and is null 

 at the intermediate periods. It is then only that the frog is in- 

 active. The very great resistance which it opposes extinguishes 

 the following oscillations. In this way more than fifty equidistant 

 oscillations have been observed. 



Schiller connected the armatures of a condenser with a quadrant 

 electrometer, one of the armatures and the corresponding pair of 

 quadrants being to earth. The first contact of the pendulum breaks 

 the inducing circuit, the second separates the induced wire from the 

 insulated armature of the condenser. The electrometer gives then 

 the difference of potential of the two armatures at the instant of the 

 second contact. The micrometric screw of the second lever enables 

 us to measure intervals of time of 0*000001 of a second. 



912. When the variable current is naturally periodic, as that 

 of most machines based on induction, or if by any device we can 

 make the effects periodic, instead of working on a single period, 

 it is more advantageous to arrange the break so as to catch the 

 effect we want to measure at a given, and always the same, time 

 of the successive periods. We may thus substitute the measure 

 of a permanent effect for that of a temporary one. 



Such are the experiments of Guillemin* on the current on 

 closing in telegraph lines. 



A rotating cylinder of wood has on its periphery a series of 

 studs or plates of copper of different breadths, which by means 

 of elastic plates render it possible to close the current for known 



* C. M. GUILLEMIN. Ann. de Chim. et de Phys. [3], Vol. LX., p. 385. 1860. 



