106 APPENDIX I. 



broken. This circuit comprises a battery B 2 , and 

 the primary coil p c of an induction apparatus, from 

 whose secondary coil s c wires extend to the part of 

 the nerve which is to be stimulated, or to the muscle. 

 No perceptible time elapses between the breaking of 

 the primary circuit and the generation of the in- 

 duced current, and the duration of the latter does 

 not exceed a few ten-thousandths of a second. 

 Hence the stimulation of the nerve can be assumed 

 to happen at the very instant the chronoscopic cir- 

 cuit is made. 



By means of the screw s, it is easy to make the 

 muscle support the lever / k so that the platinum 

 point just rests on the plate p. This is done by 

 lowering the screw, till on percussing the lever 

 above the platinum point no clattering is heard. If 

 now the tension of the muscle be ever so little in- 

 creased, the lever will be lifted, and the chronoscopic 

 circuit broken at p. 



After the contraction is over, as the lever sinks 

 back into its original position, the chronoscopic cir- 

 cuit would be made again, and the experiment spoilt 

 in consequence of the new and overpowering action 

 exercised on the needle, unless some measure were 

 taken to prevent it. This very serious difficulty has 

 been met by Professor Helmholtz with singular 

 felicity. The mercury cup into which the amalga- 

 mated copper point dips is lowered before the ex- 

 periment, till by capillary attraction the mercury is 

 drawn up in a cone connected with the point by a 

 thread. The slightest upward motion of the point 

 then will cause the mercury thread to break, after 



