410 GENERAL PHYSIOLOGY 



increase in the short circuit, the more does the current that passes 

 through the circuit of the preparation increase in intensity, and, 

 since the resistances are measured exactly, the intensity of the 

 latter current can be graduated very delicately. 



Finally, we must consider the methods of allowing a current of 

 momentary duration to act upon the preparation, and of producing 

 currents of momentary duration in rapid rhythmic succession. 

 These are presented by the phenomena of induction. 



If two coils of wire are in the same vicinity but not in contact with 

 one another, and if a constant current be allowed to flow through 

 one, the so-called primary coil (Fig. 194), at the moment of the 

 making of this primary current there appears a current in the 

 second, the secondary coil. This induced current is of very 

 brief duration ; it exists at the moment of making the primary 

 current, but disappears at once. So long as the primary current 

 passes through the primary coil, not the slightest current is 



N 



FIG. 194. Scheme of induced current. I, Primary coil ; , element ; S, key. II, Secondary 



coil ; N, preparation. 



present in the secondary coil ; but a brief induced current 

 appears again in the secondary coil as soon as the primary current 

 is broken. Thus, an induced current appears only at the moments 

 of making and breaking the primary current. The making 

 induced current is, however, in certain respects essentially different 

 from the breaking induced current. While the direction of the 

 former is opposite to that of the primary current, the breaking 

 induced current has the same direction as the latter. This fact is 

 important, for it explains at the same time another difference 

 between the making and breaking shocks. If the current in the 

 primary coil be made, it induces at the time of its appearance not 

 only in the secondary coil, but also in the turns of its own coil, a 

 current in the opposite direction ; and this extra current 

 running in opposition to it hinders the increase of the primary 

 current, until the latter has reached its greatest intensity, when 

 the induction-effect ceases. The case is different at the breaking 

 of the primary current, for the extra current that then appears in the 

 turns of the primary coil, has the same direction as the primary 



