68o TEXT-BOOK OF PHYSIOLOGY. 



moved toward or away from the primary. The distance between the two 

 coils can be measured and the strength of the induced current again re- 

 produced, other things being equal, by means of a centimeter-millimeter 

 scale pasted on the edge of B. 



The ends of the wire of the secondary coil are fastened to two binding 

 posts to which conducting wires provided with hand electrodes can be 

 attached. 



The inductorium may be used for obtaining either a single current or 

 a series of rapidly repeated induced currents. 



The Single Induced Current. On account of its high electro-motive 

 force, its penetrative power, and short duration, the single induced current 

 is a most convenient and suitable form of stimulus for many purposes. 

 In order to obtain such a current, the positive wire of the cell is carried to 

 binding post S", and the negative wire either to S'" or P". A key is placed 

 in the primary circuit. The course of the current will then be on the 

 closure of the circuit from the cell to S", thence around R' to S'", and so 

 back to the cell; or if the negative wire is connected with P", the course 

 of the current on leaving R' will be through the coils surrounding the two 

 vertical bars B', thence to P", and so back to the cell. If the secondary 

 coil be placed close to the primary and the wires of the secondary brought 

 into contact with a muscle, it will be found that with both the make and 

 the break of the primary circuit a current is induced in the secondary, as 

 shown by a short quick pulsation of the muscle; but during the time of 

 closure of the circuit, the induced current is wanting, as shown by the 

 quiescent condition of the muscle. It will be apparent, however, from 

 the energy of the contraction that the break induced current is a more 

 efficient stimulus than the make induced current. That this is the case 

 is made evident by removing the secondary to the end of the slideway and 

 then gradually bringing it toward the primary half a centimeter at a time, 

 making and breaking the circuit after each movement until a pulsation of 

 the muscle occurs. It will be found to occur first on the break of the 

 circuit. As the secondary approaches the primary a position will be 

 reached when a pulsation occurs on the make as well as on the break of 

 the circuit, though it will be less pronounced. 



The explanation offered for this difference in the strength of the two induced 

 currents is as follows: With the make of the circuit and the passage of the battery 

 current through the primary coil there is induced in the neighboring and parallel 

 turns of the wire an extra current opposite in direction to the primary current. This 

 extra or self-induced current antagonizes and prevents the current from attaining 

 its maximum development as quickly as it otherwise would, and therefore its efficiency 

 as an inducer of a current in the secondary is diminished. On the break of the circuit 

 the primary current disappears quickly, and as there is nothing to retard its disappear- 

 ance its efficiency as an inducer of a current in the secondary coil is not diminished. 

 It is not infrequently stated that the disappearance of the primary current induces in 

 the neighboring coils a break extra current corresponding in direction which assists 

 in the development of the induced current. This is not the case, however, as no 

 break extra current is developed in the inductorium as ordinarily used when actuated 

 by a battery current of moderate strength. 



As it is not so much the intensity of the current as it is rapid variations in intensity 

 that produce effects, it is readily apparent why the induced current developed at 

 the break of the primary is more effective as a stimulus than the induced current 

 developed at the make of the primary circuit. The quantity of the electricity is, 

 however, the same in both cases. 



