258 ON POSITIVE VARIATION OF THE NERVE CURRENT 



the electromagnet were in the primary current, extra currents 

 would be produced which it is desirable to avoid. The turns on the 

 outer ring together with those on the inner ring form the secondary 

 coil. In consequence of the close proximity of all the turns of this 

 double coil to the inducing ring of copper, even I Daniell is quite 

 sufficient to obtain tetanus from a sensitive nerve-muscle preparation. 

 In the experiments which we are going to consider, 3 Daniells 

 were used. In order to ensure the possibility of varying the 

 strength of the induction currents, the copper ring is made move- 

 able, and can be partly or entirely removed from the rings of the 

 secondary coil. 



Although this induction apparatus is preferable to the ordinary 

 one in all experiments where it is desirable to obtain an equal 

 duration of the make- and break-currents, yet it shares with it some 

 of the disadvantages connected with the mode of interruption, par- 

 ticularly that the interval between the make and the following 

 break is not quite equal to that between the break and the follow- 

 ing make, unless a special contrivance is used for the purpose. 

 Moreover, exact equality between make- and break-currents was not 

 obtained. 



The apparatus I use is only intended for the stimulation 

 of nerve. The strength of the currents can however be in- 

 creased, with the same number of turns, by increasing the diameter 

 of the rings. I also stimulated with ascending and descending 

 break-currents of an ordinary induction apparatus, the make currents 

 being cut off. Finally, interrupted battery-currents ascending, 

 descending, and alternately in opposite directions were employed 

 to stimulate the nerve. In all cases the positive variation of the 

 nerve current was clearly demonstrated. All experiments were 

 made with a Wiedemann's galvanometer with the magnet aperiodic. 

 This instrument is not especially well adapted to these experiments, 

 because the time which the magnet takes in coming to rest is too 

 great in proportion to the duration of the positive variation. I shall 

 take some opportunity of repeating the experiments with a galvano- 

 meter more suited to the purpose. So much of the phenomenon 

 as I am now concerned with can be satisfactorily examined with the 

 above-named galvanometer. 



