460 



NER VE. 



consequence of comparatively large electromotive force, brief duration, 

 and rapidity of development, they form the most effective of all nerve 

 stimuli. 



Their employment necessitates the adoption of certain precautions due to 

 their character and method of production. These currents are largely modified 

 by the presence of the extra current in the primary coil, and therefore by the 

 nature of this coil ; if, as in most induction apparatus, this contains a core, 

 then such modification is very pronounced. Under any circumstances the 

 " make " induced effect is slower in its development and subsidence than the 

 " break " (Fig. 252, A). This is due to the variation in the counteracting influ- 

 ence of the extra current, 1 which, on closing the primary circuit, is of some 

 intensity, but on opening is comparatively feeble, since, in the latter case, the 

 circuit being opened, it must traverse the gap as a spark. The time-relations 

 of the break induced current are thus dependent upon the velocity with which 

 the key in the primary circuit is opened. It will be seen that rapidity of 

 development is an important factor in determining the stimulating intensity 

 of a current ; hence, in the case of the break induced current, its excitation 

 value is dependent upon the rate at which the circuit of the primary coil is 

 opened. 



For many experiments it is necessary that the time-relations of each one of 



a series of stimulating in- 

 duced currents should be 

 absolutely uniform ; to 

 insure this, a key must be 

 used, such that the primary 

 circuit shall be always 

 opened at a constant velo- 

 city. If all other condi- 

 tions are constant, then the 

 more quickly the primary 

 c i rcu i t i s opened, the more 

 -i -11 t ,v i i 



ra ? ld ^IH th ? deVel P; 

 ment or the break induced 



current, and the more 



powerful its stimulating effect. A large number of different forms of apparatus 

 of varying degrees of efficiency have been employed to effect this end ; a spring 

 which on its release separates two surfaces, or a traveller driven by either a 

 weight or some constant force, which opens at a uniform velocity an appropriate 

 key. 



It is less easy to secure a constant closure, hence the break is to be pre- 

 ferred to the make induced current. 



A useful and sufficiently uniform stimulus is that produced by a closure 

 of the primary circuit for a definite but very short period, such as can be 

 effected by the striking of a steel needle as it passes over and lightly touches 

 a stretched wire ; this is made use of in many forms of rheotome, and, when 

 produced by a rotator travelling at a constant and sufficiently rapid velocity, 

 causes a series of uniform induced currents in the secondary circuit. 



The production of such a rapid series of uniform induced currents is 

 ordinarily effected by the use of the spring hammer of the inductorium, 

 which causes make and break induced currents recurring at regular 

 intervals. The objection that the fact of the break being different in 

 time of development to the make effect, must, when both rapidly recur, cause 

 an overpowering series of currents in one direction, is got rid of by the 

 interposition of a side wire, which is so disposed that the interrupter opens and 

 closes a short circuit of the primary coil. Under these circumstances the 

 1 See Rosenthal, " Electricitatslehre f. prakt. Aerzte." 



FIG. 252. 

 induced 



Time relations of make (w) and break (b] 

 currents (ordinates = current intensity, 

 S, Time relations^ induced currents obtained by 

 Helmholtz side wire method. C. Time relations of 

 magneto-induced currents obtained by rapid rota- 

 tion machine. 



