APPENDIX 155 



an external voltage effective for only 0.005 sec. ; in this manner 

 Gotch and Burch have assigned 0.03 volt as the electromotive value 

 of a single nervous impulse. And this is not yet a true value, by 

 reason of the internal derivation alluded to above. 



The principal object of a standard voltage at the commence- 

 ment and termination of experiment is (i) to indicate that the 

 resistance has or has not sensibly altered during experiment ; and 

 (2) to indicate the sensibility of instruments employed. It does 

 not afford in itself satisfactory data for a comparison between 

 electromotive values of response of different tissues, and it is only 

 with reservation (i.e., after control of altered resistance and altered 

 duration of action) that it can be utilised for the detection and 

 estimation of altered electromotive values during any one obser- 

 vation. 



Bearing these reservations in mind, we may, however, be 

 allowed to speak of the " voltage " of an injury current as 

 "measured" by that of a compensation current,* and to indicate 

 by reference to standard deflections the scale of voltage in which 

 blaze-currents are externally manifested. 



The exciting apparatus. In the great proportion of experiments 

 the du Bois-Reymond induction coil (Berne model) is used. This 

 coil has its graduation in arithmetical progression. 



In those experiments where it is necessary to estimate the 

 quantity or energy of the discharge, a condenser must be used. 

 In either case there are two possible methods of excitation : 

 (i) the excitation is sent through object and galvanometer in 

 series, and (2) the galvanometer is short-circuited during excita- 

 tion, and put into circuit after a given short interval of time. 



Electrodes. It is absolutely essential that the electrodes should 

 be unpolarisable. Du Bois - Reymond's combination (zinc and 

 sulphate of zinc) has, in my experience, given better electrodes 

 than those recently introduced e.g., silver and silver chloride, 

 or mercury and calomel. 



Keyboard. In the constant use of a galvanometer as a measuring 

 instrument, the circuit should be set up in such a manner that the 

 direction of exciting and reaction currents may be quickly determined. 



' The instrument by which this is done 5s a compensator ; it is not strictly speaking 

 a potentiometer \ 



